Patent Publication Number: US-6705263-B2

Title: Handheld type four-cycle engine

Description:
This application is a Divisional of prior application Ser. No. 09/803,663 filed Mar. 12, 2001 now U.S. Pat. No. 6,508,224. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to handheld type four-cycle engines which are mainly used as a power source for machines for portable operation such as trimmers. More particularly, it relates to improvement of a four-cycle engine that includes an engine main body, the engine main body including a crankcase having a crank chamber, a cylinder block having a cylinder bore and a cylinder head having an intake port and an exhaust port; a crankshaft supported in the crankcase and housed inside the crank chamber; a piston fitted in the cylinder bore and connected to the crankshaft; an intake valve and an exhaust valve for opening and closing the intake port and exhaust port, the intake valve and exhaust valve being mounted in the cylinder head; a valve operation mechanism operable in association with the rotation of the crankshaft so as to open and close the intake valve and exhaust valve; and a power output or takeoff mechanism provided on one end of the crankshaft, the end projecting out of the engine main body. 
     2. Description of the Prior Art 
     Such a handheld type four-cycle engine is already known as disclosed in, for example, Japanese Patent Application Laid-open No. 10-288019. 
     Handheld type four-cycle engines are of course useful in terms of the prevention of environmental pollution as well as assuring the operators&#39; health since the exhaust gas is comparatively clean. However, since the structure thereof is more complicated than that of two-cycle engines, there is a drawback that it is difficult to reduce the weight thereof. Weight reduction is an important issue for improvements particularly in the operability of handheld four-cycle engines. 
     However, in the handheld type four-cycle engine disclosed in the above-mentioned patent publication, a valve operation mechanism for opening and closing intake and exhaust valves provided in the upper part of a cylinder head is of a type that includes pushrods and rocker arms, and a valve operation chamber for housing the pushrods, a camshaft for driving the pushrods, etc. is formed in a side wall of the engine main body; the size of the engine main body therefore inevitably increases thus making it difficult to reduce the weight of the engine. 
     SUMMARY OF THE INVENTION 
     The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to provide a lightweight handheld type four-cycle engine having good operability by making the engine main body compact. 
     In accordance with a first characteristic of the present invention in order to achieve the above-mentioned objective, there is proposed a handheld type four-cycle engine including an engine main body, the engine main body including a crankcase having a crank chamber, a cylinder block having a cylinder bore and a cylinder head having an intake port and an exhaust port; a crankshaft supported in the crankcase and housed inside the crank chamber; a piston fitted inside the cylinder bore and connected to the crankshaft; an intake valve and an exhaust valve for opening and closing the intake port and exhaust port, the intake valve and the exhaust valve being mounted in the cylinder head; a valve operation mechanism operable in association with the rotation of the crankshaft so as to open and close the intake valve and the exhaust valve; and a power output mechanism provided on one end of the crankshaft projecting out of the engine main body, wherein the valve operation mechanism includes a camshaft rotatably supported in the cylinder head so as to open and close the intake valve and the exhaust valve, and a dry type timing transmission placed outside the engine main body on the side opposite the power output mechanism and providing association between the crankshaft and the camshaft. 
     The above-mentioned power output mechanism corresponds to the centrifugal clutch described in the embodiments below. 
     In accordance with the above-mentioned first characteristic, since the timing transmission and the power output mechanism are mounted on either side of the cylinder head on the two ends of the crankshaft, the weight balance at the two ends of the crankshaft is improved, the centre of gravity of the engine can be made as close to the central part of the crankshaft as possible, which, together with the reduced weight, can enhance the operability of the engine. Furthermore, since the loads arising from the timing transmission and the drive shaft separately work on the two ends of the crankshaft during operation of the engine so avoiding the load on the crankshaft and its bearings from being localised, the durability thereof can be enhanced. 
     In accordance with a second characteristic of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the timing transmission is made as a dry type and is separate from the crank chamber. 
     In accordance with the above-mentioned second characteristic, since it is unnecessary to provide the side wall of the engine main body with a special chamber for housing the timing transmission, the engine main body can be made thinner and more compact thus achieving a large reduction in the weight of the entire engine. 
     In accordance with a third characteristic of the present invention, in addition to the above-mentioned first or second characteristic, there is proposed a handheld type four-cycle engine wherein a flywheel is mounted on the crankshaft between the engine main body and the power output mechanism, the flywheel including cooling vanes for sending cooling air to the engine main body and having a diameter larger than that of the power output mechanism. 
     In accordance with the above-mentioned third characteristic, the cooling air can be supplied appropriately to the engine main body, without obstruction from the power output mechanism, by rotation of the cooling vanes while minimising any increase in the size of the engine due to the flywheel, and the cooling performance thereof can be enhanced. 
     In accordance with a fourth characteristic of the present invention, in addition to the above-mentioned first or second characteristic, there is proposed a handheld type four-cycle engine wherein an oil tank for storing a lubricating oil for lubricating the inside of the engine main body is placed outside the timing transmission so as to adjoin it and is supported on the engine main body. 
     In accordance with the above-mentioned fourth characteristic, since the oil tank covers at least one part of the timing transmission, the transmission can be protected. Moreover, since the oil tank and the flywheel are positioned opposite to each other, the centre of gravity of the engine can be made as close to the central part of the crankshaft as possible and the operability of the engine can be further enhanced. 
     In accordance with a fifth characteristic of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the valve operation mechanism includes the timing transmission placed outside the engine main body and linked to one end of the crankshaft and a cam system for transmitting the rotational force of the driven side of the timing transmission to the intake and exhaust valves for opening and closing forces, a first valve mechanism chamber housing the timing transmission is provided integrally with an oil tank that is placed outside of the engine main body on the same side as the timing transmission, a second valve mechanism chamber housing at least one part of the cam system is formed in the cylinder head, and a pair of oil slingers for stirring and scattering the oil stored in the oil tank in order to generate an oil mist that is to be supplied to the second valve operation chamber and the crank chamber are fixed to the crankshaft so that the timing transmission is interposed between the pair of the slingers. 
     In accordance with the above-mentioned fifth characteristic, since the oil tank is placed on one side outside the engine main body, the total height of the engine can be greatly reduced. Moreover, since the first valve operation chamber housing the timing transmission is provided integrally with the oil tank, one part of the timing transmission is housed in the oil tank so making the engine more compact. 
     Furthermore, since the lubrication system of the valve operation mechanism is divided into two parts, that is, a part for lubricating the timing transmission inside the first valve operation chamber with the oil scattered inside the oil tank, and a part for lubricating the cam system inside the second valve operation chamber with the oil mist generated inside the oil tank, the load put on each part of the lubrication system is lessened and the entire valve operation mechanism can be lubricated thoroughly. 
     Moreover, the pair of the oil slingers are fixed to the crankshaft with the timing transmission is placed therebetween, the oil stored inside the oil tank can be stirred and scattered without obstruction from the timing transmission regardless of the operational position of the engine and the oil mist can be generated effectively. 
     Furthermore, in accordance with a sixth characteristic of the present invention, in addition to the above-mentioned fifth characteristic, there is proposed a handheld type four-cycle engine wherein a through hole through which the oil mist generated in the oil tank is supplied to the crank chamber is provided in the crankshaft, and an open end of the through hole in the oil tank is positioned between the timing transmission and an oil slinger. 
     In accordance with the above-mentioned sixth characteristic, the open end of the through hole of the crankshaft can be positioned in the central area of the oil tank or in the vicinity thereof without obstruction from the timing transmission or the oil singers, and it is possible to prevent the oil stored inside the oil tank from entering the through hole directly. 
     Furthermore, in accordance with a seventh characteristic of the present invention, in addition to the above-mentioned fifth characteristic, there is proposed a handheld type four-cycle engine wherein the oil tank for storing lubricating oil and the timing transmission of the valve operation mechanism are placed on one side of the engine main body, the timing transmission extending into the oil tank, a belt guide tube housing the timing transmission is provided integrally with the oil tank, and the open end of the belt guide tube inside the oil tank projects towards the central part of the oil tank so that the open end is above the liquid level of the stored oil regardless of whether the engine is upside down or laid on its side. 
     In accordance with the above-mentioned seventh characteristic, the total height of the engine can be reduced, at the same time any increase in the width of the engine can be minimised, and the engine can therefore be made more compact. Moreover, since the open end inside the oil tank of the belt guide tube housing the timing transmission is always above the liquid level of the stored oil even when the engine is upside down or laid on its side, the stored oil is prevented from flowing towards the timing transmission, oversupply of oil to the timing transmission can be prevented and at the same time the amount of oil stored in the oil tank can be maintained at a predetermined level. 
     Furthermore, in accordance with an eighth characteristic of the present invention, in addition to the above-mentioned fifth characteristic, there is proposed a handheld type four-cycle engine wherein the oil tank, an end of the crankshaft extending into the oil tank, and the timing transmission of the valve operation mechanism linked to the crankshaft inside the oil tank are placed outside the engine main body on the side opposite to the power output mechanism, and the timing transmission is lubricated by the oil inside the oil tank. 
     In accordance with the above-mentioned eighth characteristic, it is unnecessary to provide a special chamber for housing the timing transmission in the side wall itself of the engine main body, the total height of the engine can be reduced due to the sideways arrangement of the oil tank, the side wall of the engine main body can thus be made thinner and more compact, and the weight of the entire engine can be greatly reduced. Moreover, the weight balance at the two ends of the crankshaft is improved by placing the power output mechanism on one side of the engine main body and the timing transmission and the oil tank on the other side, the centre of gravity of the engine can be made as close to the central part of the crankshaft as possible, which, together with the reduced weight, can enhance the operability of the engine. 
     Moreover, since the loads arising from the timing transmission and the power output mechanism during operation of the engine separately work on the two ends of the crankshaft so avoiding the load on the crankshaft and its bearings from being localised, the durability thereof can be enhanced. 
     Furthermore, since the timing transmission is lubricated directly with oil inside the oil tank, the lubrication system can be simplified. 
     In accordance with a ninth characteristic of the present invention, in addition to the above-mentioned eighth characteristic, there is proposed a handheld type four-cycle engine wherein a cooling fan is fixed to the crankshaft between the engine main body and the power output mechanism, the cooling fan having a diameter larger than that of the power output mechanism. 
     In accordance with the above-mentioned ninth characteristic, any increase in size of the engine can be minimised while enhancing the air supply performance of the cooling fan. 
     In accordance with a tenth characteristic of the present invention, in addition to the above-mentioned eighth characteristic, there is proposed a handheld type four-cycle engine wherein the cam system for transmitting the rotation of the driven side of the timing transmission to the intake valve and the exhaust valve for opening and closing forces is placed in the valve operation chamber provided in the cylinder head, and oil mist generation means for generating an oil mist inside the oil tank is linked to the crankshaft, the oil mist being supplied to the valve operation chamber. 
     In accordance with the above-mentioned tenth characteristic, since the lubrication system of the valve operation mechanism is divided into two part, that is, a part for lubricating the timing transmission with oil inside the oil tank, and a part for lubricating the cam system with oil mist generated inside the oil tank, the load put on each part of the lubrication system is lessened and the entire valve operation mechanism can be lubricated thoroughly. 
     Furthermore, in accordance with an eleventh characteristic of the present invention, in addition to the above-mentioned fifth characteristic, there is proposed a handheld type four-cycle engine the timing transmission of the valve operation mechanism is constructed as a wrap-around type having a wrap-around member, the drive side of the wrap-around member extending into the oil tank, oil mist generation means for generating an oil mist for lubricating the timing transmission by scattering oil stored inside the oil tank is provided in the oil tank, and an oil droplet guide wall is provided so as to project out of the inner wall of the oil tank, the oil droplet guide wall guiding and dripping the attached oil droplets onto the part of the timing transmission extending into the oil tank when the engine is laid on its side. 
     In accordance with the above-mentioned eleventh characteristic, when the engine is operated in a laid-sideways state, the oil mist attached to the oil droplet guide wall turns into oil droplets, the droplets then fall down onto the wrap-around member on the drive side of the timing transmission and, in particular, when the upper part of the wrap-around member moves from the drive side to the driven side, the above-mentioned oil droplets can be carried by the wrap-around member to the driven side with hardly any influence from centrifugal force and the driven side can be lubricated reliably. 
     The above-mentioned wrap round member corresponds to the timing belt  25 ,  125 ,  225  in the embodiments of the present invention described below. 
     Furthermore, in accordance with a twelfth characteristic of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the valve operation mechanism is provided over an oil tank placed outside the engine main body and storing a lubricating oil, a first valve operation chamber formed so as to extend upwards from the oil tank, and a second valve operation chamber formed in the cylinder head; the oil tank and the crank chamber are communicated with each other by means of a through hole; the crank chamber and the second valve operation chamber are communicated with each other by means of an oil feed pipe provided outside the engine main body; the second valve operation chamber and the oil tank are communicated with each other by means of an oil return passage; the oil tank includes oil mist generation means for generating an oil mist by stirring and scattering the stored oil; and transfer means for transferring the oil mist inside the oil tank to the oil feed pipe via the crank chamber is connected to the oil feed pipe so that the valve operation mechanism inside the first valve operation chamber is lubricated with the oil scattered inside the oil tank; and the valve operation mechanism inside the second valve operation chamber is lubricated with oil mist transferred from the oil feed pipe to the second valve operation chamber. 
     In accordance with the twelfth characteristic of the present invention, since the oil feed pipe is placed outside the engine main body, it is possible to make the side wall of the engine main body thinner regardless of the presence of the pipe, the engine main body can be made more compact and the weight of the entire engine can thus be reduced. Moreover, since the oil feed pipe outside the engine main body easily radiates heat, cooling of the oil mist passing through the pipe can be improved. 
     Since the lubrication system of the valve operation mechanism is divided into two parts, that is, a part for lubricating the valve operation mechanism inside the oil tank and the first valve operation chamber with the oil scattered inside the oil tank, and a system for lubricating the valve operation mechanism inside the second valve operation chamber with the oil mist transferred to the second valve operation chamber, the load put on each part of the lubrication system is lessened and the entire valve operation mechanism can be lubricated thoroughly. 
     Moreover, each part of the engine can be lubricated reliably regardless of the operational position of the engine by the use of oil droplets and oil mist. 
     Furthermore, in accordance with a thirteenth characteristic of the present invention, in addition to the above-mentioned twelfth characteristic, there is proposed a handheld type four-cycle engine wherein the transfer means includes valve means that closes the oil feed pipe when the pressure of the crank chamber is negative and opens the pipe when the pressure is positive. 
     In accordance with the thirteenth characteristic of the present invention, it is unnecessary to employ a special oil pump for circulating the oil mist, and the structure can be simplified. 
     Furthermore, in accordance with a fourteenth characteristic of the present invention, in addition to the above-mentioned twelfth or thirteenth characteristic, there is proposed a handheld type four-cycle engine wherein the oil feed pipe and the oil return passage are connected to each other via a bypass. 
     In accordance with the above-mentioned fourteenth characteristic, the amount of oil mist supplied to the second valve operation chamber from the oil feed pipe can be controlled by appropriately selecting the flow resistance of the bypass. 
     In accordance with a fifteenth characteristic of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the valve operation mechanism includes the timing transmission which has a rotating drive member linked to the crankshaft and a cam system for transmitting the rotational force of a rotating driven member of the timing transmission to the intake valve and exhaust valve for opening and closing forces; a first valve operation chamber and an oil tank are provided on one side of the engine main body, the first valve operation chamber housing the timing transmission, the oil tank including oil mist generation means for generating an oil mist from stored oil, and the lower end of the first valve operation chamber opening inside the oil tank; a second valve operation chamber housing the cam system is provided in the upper part of the engine main body so as to be in line with the first valve operation chamber; a first lubrication system includes first and second oil passages placed alongside each other and providing communication between the oil tank and the crank chamber, and first oil feed means for circulating the oil mist generated inside the oil tank from the oil tank via the first oil passage, the crank chamber, and the second oil passage, back to the oil tank; and a second lubrication system includes a third oil passage providing communication between the first valve operation chamber and the second valve operation chamber, a fourth oil passage providing communication between the second valve operation chamber and the crank chamber, the second oil passage, and second oil feed means for circulating the oil mist generated inside the oil tank from the oil tank via the first valve operation chamber, the third oil passage, the second valve operation chamber, the fourth oil passage, the crank chamber, and the second oil passage, back to the oil tank. The rotating drive member and the rotating driven member correspond to the drive pulley  223  and the driven pulley  224  of the third embodiment of the present invention described below, and the oil mist generation means corresponds to the oil slingers  256   a  and  256   b.    
     In accordance with the fifteenth characteristic, since the surroundings of the crank shaft are lubricated by the first and second lubrication systems, and the timing transmission and the cam system of the valve operation system are lubricated by the second lubrication system, the circumference of the crankshaft which is subjected to a comparatively high load can be lubricated adequately, at the same time it is possible to prevent excessive lubrication of the valve operation mechanism which is subjected to a comparatively low load, the amount of oil mist circulated can be minimised, the amount of oil stored in the oil tank can be reduced, and not only the oil tank but also the entire engine can be made more compact and lighter. 
     In accordance with a sixteenth characteristic of the present invention, in addition to the above-mentioned fifteenth characteristic, there is proposed a handheld type four-cycle engine wherein the first oil feed means includes a first one-way valve provided in the second oil passage, closing when the pressure of the crank chamber decreases and opening when the pressure increases, and the second oil feed means includes a second one-way valve provided in the third oil passage, closing when the pressure of the crank chamber decreases and opening when the pressure increases. 
     In accordance with the sixteenth characteristic, the oil mist inside the oil tank can be circulated by utilising the pressure pulsations within the crank chamber and the one-way transfer functions of the first and second one-way valves, it is therefore unnecessary to employ a special oil pump for circulation of the oil mist and the structure can thus be simplified. 
     The above-mentioned objects, other objects, characteristics and advantages of the present invention will become apparent from an explanation of preferable embodiments which will be described in detail below by reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  11  show a first embodiment of the present invention. 
     FIG. 1 is an oblique view showing one embodiment of the handheld type four-cycle engine of the present invention in practical use. 
     FIG. 2 is a longitudinal side view of the above-mentioned four-cycle engine. 
     FIG. 3 is a enlarged view of an essential part of FIG.  2 . 
     FIG. 4 is a enlarged vertically sectioned view around the camshaft in FIG.  3 . 
     FIG. 5 is a cross-sectional view at line  5 — 5  in FIG.  3 . 
     FIG. 6 is a schematic view of the lubrication system of the above-mentioned engine. 
     FIG. 7 is a cross-sectional view at line  7 — 7  in FIG.  3 . 
     FIG. 8 is a cross-sectional view at line  8 — 8  in FIG.  7 . 
     FIG. 9 is a bottom view of the head cover. 
     FIG. 10 is an explanatory view of the action of the suction of the oil residing in the cylinder head in various operational positions of the engine. 
     FIG. 11 is a cross-sectional view corresponding to FIG. 7, showing a modified embodiment of the oil feed pipe and oil return pipe. 
     FIGS. 12 to  24  show a second embodiment of the present invention. 
     FIG. 12 is a longitudinal side view of the handheld type four-cycle engine of the present invention. 
     FIG. 13 is a cross-sectional view at line  13 — 13  in FIG.  12 . 
     FIG. 14 is a cross-sectional view at line  14 — 14  in FIG.  12 . 
     FIG. 15 is a enlarged cross-sectional view of an essential part of FIG.  12 . 
     FIG. 16 is a exploded view of an essential part of FIG.  15 . 
     FIG. 17 is a cross-sectional view at line  17 — 17  in FIG.  14 . 
     FIG. 18 is a cross-sectional view at line  18 — 18  in FIG.  14 . 
     FIG. 19 is a cross-sectional view at line  19 — 19  in FIG.  18 . 
     FIG. 20 is a cross-sectional view at line  20 — 20  in FIG. 15 (view of the head cover from below). 
     FIG. 21 is a cross-sectional view at line  21 — 21  in FIG.  15 . 
     FIG. 22 is a diagram showing the lubrication route of the above-mentioned engine. 
     FIG. 23 is a view corresponding to FIG. 14 in which the above-mentioned engine is in an upside down state. 
     FIG. 24 is a view corresponding to FIG. 14 in which the above-mentioned engine is in a laid-sideways state. 
     FIGS. 25 to  36  show a third embodiment of the present invention. 
     FIG. 25 is a longitudinal side view of the handheld type four-cycle engine of the present invention. 
     FIG. 26 is a cross-sectional view at line  26 — 26  in FIG.  25 . 
     FIG. 27 is a cross-sectional view at line  27 — 27  in FIG.  25 . 
     FIG. 28 is a enlarged cross-sectional view of an essential part of FIG.  25 . 
     FIG. 29 is a exploded view of an essential part of FIG.  28 . 
     FIG. 30 is a cross-sectional view at line  30 — 30  in FIG.  27 . 
     FIG. 31 is a cross-sectional view at line  31 — 31  in FIG.  27 . 
     FIG. 32 is a cross-sectional view at line  32 — 32  in FIG. 28 (view of the head cover from below). 
     FIG. 33 is a cross-sectional view at line  33 — 33  in FIG.  28 . 
     FIG. 34 is a diagram showing the lubrication route of the above-mentioned engine. 
     FIG. 35 is a view corresponding to FIG. 27 in which the above-mentioned engine is in an upside down state. 
     FIG. 36 is a view corresponding to FIG. 27 in which the above-mentioned engine is in a laid-sideways state. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Firstly, the first embodiment of the present invention shown in FIGS. 1 to  11  is explained below. 
     As shown in FIG. 1, a handheld type four-cycle engine E is attached as a source of power to the drive section of, for example, a powered trimmer T. Since the powered trimmer T is used in a manner in which a cutter C is positioned in various directions according to the operational conditions, the engine E is also tilted to a large extent or turned upside-down, and as a result and the operational position is unstable. 
     First of all, the overall construction of the handheld type four-cycle engine is explained by reference to FIGS. 2 to  5 . 
     As shown in FIGS. 2,  3  and  5 , a carburettor  2  and an exhaust muffler  3  are attached to the front and back respectively of an engine main body  1  of the above-mentioned handheld type four-cycle engine E, and an air cleaner  4  is attached to the inlet of the carburettor  2 . A fuel tank  5  made of a synthetic resin is attached to the lower face of the engine main body  1 . 
     The engine main body  1  includes a crankcase  6  having a crank chamber  6   a , a cylinder block  7  having one cylinder bore  7   a , and a cylinder head  8  having a combustion chamber  8   a  and intake and exhaust ports  9  and  10  which open into the combustion chamber  8   a . The cylinder block  7  and the cylinder head  8  are integrally cast, and the separately cast crankcase  6  is bolt-joined to the lower end of the cylinder block  7 . The crankcase  6  is formed from first and second case halves  6 L and  6 R, and the two case halves  6 L and  6 R are joined to each other by means of a bolt  12  in the centre of the crankcase  6 . A large number of cooling fins  38  are formed on the outer peripheries of the cylinder block  7  and the cylinder head  8 . 
     A crankshaft  13  housed in the crank chamber  6   a  is rotatably supported in the first and second case halves  6 L and  6 R via ball bearings  14  and  14 ′, and is connected to a piston  15  fitted in the cylinder bore  7   a  via a connecting rod  16 . Moreover, oil seals  17  and  17 ′ are fitted in the first and second case halves  6 L and  6 R, the oil seals  17  and  17 ′ adjoining the above-mentioned bearings  14  and  14 ′ and being in close contact with the outer circumference of the crankshaft  13 . 
     An intake valve  18  and an exhaust valve  19  for opening and closing the intake port  9  and the exhaust port  10  respectively are provided in the cylinder head  8  parallel to the axis of the cylinder bore  7   a , and a spark plug  20  is screwed in so that the electrodes thereof are close to the central area of the combustion chamber  8   a.    
     The intake valve  18  and the exhaust valve  19  are forcedly closed by means of valve springs  22  and  23  in a valve cam operation chamber  21  formed in the cylinder head  8 . In the valve cam operation chamber  21 , cam followers  24  and  25  supported in the cylinder head  8  in a vertically rockable manner are superimposed on top of the intake valve  18  and the exhaust valve  19 , and a camshaft  26  for opening and closing the intake valve  18  and the exhaust valve  19  via the cam followers  24  and  25  is rotatably supported via ball bearings  27 ′ and  27  in the right and left side walls of the valve cam operation chamber  21 , the camshaft  26  being parallel to the crankshaft  13 . One side wall of the valve cam operation chamber  21  in which the bearing  27  is mounted is formed integrally with the cylinder head  8 , and an oil seal  28  is mounted in this side wall in close contact with the outer circumference of the camshaft  26 . The other side wall of the valve cam operation chamber  21  is provided with an insertion opening  29  to allow the camshaft  26  to be inserted into the valve cam operation chamber  21 , and after inserting the camshaft  26  the other bearing  27 ′ is mounted in a side wall cap  30  that blocks the insertion opening  29 . The side wall cap  30  is fitted in the insertion opening  29  via a sealing member  31  and joined to the cylinder head  8  by means of a bolt. 
     As is clearly shown in FIGS. 3 and 4, one end of the camshaft  26  projects out of the cylinder head  8  on the side of the above-mentioned oil seal  28 . One end of the crankshaft  13  also projects out of the crankcase  6  on the same side, a toothed drive pulley  32  is fixed to this end of the crankshaft  13 , and a toothed driven pulley  33  having twice as many teeth as that of the drive pulley  32  is fixed to the end of the above-mentioned camshaft  26 . A toothed timing belt  34  is wrapped around the two pulleys  32  and  33  so that the crankshaft  13  can drive the camshaft  26  at a reduction rate of ½. The above-mentioned camshaft  26  and a timing transmission  35  form a valve operation mechanism  53 . 
     The engine E is thus arranged in the form of an OHC type, and the timing transmission  35  is in the form of a dry type which is placed outside the engine main body  1 . 
     A belt cover  36  made of a synthetic resin is placed between the engine main body  1  and the timing transmission  35 , the belt cover  36  being fixed to the engine main body  1  by means of a bolt  37 , so that the heat radiated from the engine main body  1  is prevented from affecting the timing transmission  35 . 
     An oil tank  40  made of a synthetic resin placed so as to cover a part of the outer face of the timing transmission  35  is fixed to the engine main body  1  by means of a bolt  41  and, moreover, a recoil type starter  42  (see FIG. 2) is fitted to the outer face of the oil tank  40 . 
     Referring again to FIG. 2, the end of the crankshaft  13  opposite to the end of the timing transmission  35  also projects out of the crankcase  6 , and a flywheel  43  is fixed to the end by means of a nut  44 . A large number of cooling vanes  45 ,  45  . . . are integrally provided on the inner face of the flywheel  43  so that the flywheel  43  can also function as cooling means. A plurality of fitting bosses  46  (one thereof is shown in FIG. 2) are formed on the outer face of the flywheel  43 , and a centrifugal shoe  47  is pivotally supported on each of the fitting bosses  46 . These centrifugal shoes  47 , together with a clutch drum  48  fixed to a drive shaft  50  which will be described below, form a centrifugal clutch  49 , and when the rotational rate of the crankshaft  13  exceeds a predetermined value, the centrifugal shoes  47  are pressed onto the inner periphery of the clutch drum  48  due to the centrifugal force of the shoes so transmitting the output torque of the crankshaft  13  to the drive shaft  50 . The flywheel  43  has a larger diameter than that of the centrifugal clutch  49 . 
     An engine cover  51  covering the engine main body  1  and its attachments is divided at the position of the timing transmission  35  into a first cover half  51   a  on the side of the flywheel  43  and a second cover half  51   b  on the side of the starter  42 , and each of the cover halves  51   a  and  51   b  is fixed to the engine main body  1 . A truncated cone shaped bearing holder  58  coaxially arranged with the crankshaft  13  is fixed to the first cover half  51   a , the bearing holder  58  supporting the drive shaft  50  which rotates the above-mentioned cutter C via a rotating bearing  59 , and an air intake opening  52  is provided in the bearing holder  58  so that outside air is drawn inside the engine cover  51  by rotation of the cooling vanes  45 ,  45  . . . . Furthermore, a base  54  for covering the lower face of the fuel tank  5  is fixed to the engine cover  51  and the bearing holder  58 . 
     As mentioned above, since the timing transmission  35  for operatively connecting the crankshaft  13  to the camshaft  26  is constructed as a dry type outside the engine main body  1 , it is unnecessary to provide a special compartment for housing the transmission  35  on the side wall of the engine main body  1  and it is therefore possible to make the engine main body  1  thin and compact and greatly reduce the overall weight of the engine E. 
     Moreover, since the timing transmission  35  and the centrifugal shoes  47  of the centrifugal clutch  49  are connected to the two ends of the crankshaft  13  with the cylinder block  7  interposed between them, the weights at the two ends of the crankshaft  13  are well balanced, the centre of gravity of the engine E can be set as close to the central part of the crankshaft  13  as possible, and the operability of the engine E can thus be enhanced while reducing the weight. Furthermore, since the loads from the timing transmission  35  and the drive shaft  50  separately work on the two ends of the crankshaft  13  during operation of the engine E, it is possible to prevent the load on the crankshaft  13  and the bearings  14  and  14 ′ supporting the crankshaft  13  from being localised and the durability thereof can thus be enhanced. 
     Furthermore, since the flywheel  43  having a diameter larger than that of the centrifugal clutch  49  and having the cooling vanes  45  is fixed to the crankshaft  13  between the engine main body  1  and the centrifugal clutch  49 , external air can be supplied effectively around the cylinder block  7  and the cylinder head  8  by introducing the air through the air intake opening  52  by rotation of the cooling vanes  45  without interference from the centrifugal clutch  49  thus enhancing the cooling performance while preventing any increase in the size of the engine E due to the flywheel  43 . 
     Moreover, since the oil tank  40  is fitted to the engine main body  1  so as to adjoin the outside of the timing transmission  35 , the oil tank  40  covers at least a part of the timing transmission  35  and can protect the transmission  35  in co-operation with the second cover half  51   b  covering the other part of the transmission  35 . In addition, since the oil tank  40  and the flywheel  43  are arranged so as to face each other with the engine main body  1  interposed between them, the centre of gravity of the engine E can be set closer to the central part of the crankshaft  13 . 
     The lubrication system of the above-mentioned engine E is explained below by reference to FIGS. 3 to  10 . 
     As shown in FIG. 3, the crankshaft  13  is arranged so that one end thereof runs through the oil tank  40  while being in close contact with the oil seals  39  and  39 ′ mounted in both the outside and inside walls of the oil tank  40 , and a through hole  55  providing communication between the inside of the oil tank  40  and the crank chamber  6   a  is provided in the crankshaft  13 . Lubricating oil O is stored in the oil tank  40 , and the amount stored is set so that an open end of the above-mentioned through hole  55  inside the oil tank  40  is always above the liquid level of the oil O regardless of the operational position of the engine E. 
     An oil slinger  56  is fixed to the crankshaft  13  inside the oil tank  40  by means of a nut  57 . The oil slinger  56  includes two blades  56   a  and  56   b  which extend in directions radially opposite to each other from the central part where the oil slinger  56  is fitted to the crankshaft  13 , and which are bent in directions axially opposite to each other. When the oil slinger  56  is rotated by the crank shaft  13 , at least one of the two blades  56   a  and  56   b  scatters the oil O inside the oil tank  40  so as to generate an oil mist regardless of the operational position of the engine E. 
     As shown in FIGS. 3,  6  and  7 , the crank chamber  6   a  is connected to the valve operation camber  21  via an oil feed pipe  60 , and a one-way valve  61  is provided in the oil feed pipe  60  so as to only allow flow in the direction from the crank chamber  6   a  to the valve cam operation chamber  21 . The oil feed pipe  60  is formed integrally with the aforementioned belt cover  36  along one side edge thereof, and the lower end of the oil feed pipe  60  is formed in a valve chamber  62 . An inlet pipe  63  projecting from the valve chamber  62  at the back of the belt cover  36  is formed integrally with the belt cover  36 , and the inlet pipe  63  is fitted into a connection hole  64  in the lower part of the crankcase  6  via a sealing member  65  so that the inlet pipe  63  is communicated with to provide a link to the crank chamber  6   a . The aforementioned one-way valve  61  is provided inside the valve chamber  62  so as to allow flow in the direction from the inlet pipe  63  to the valve chamber  62 . This one-way valve  61  is a reed valve in the case of the illustrated embodiment. 
     An outlet pipe  66  projecting from the upper end of the oil feed pipe  60  at the back of the belt cover  36  is formed integrally with the belt cover  36 , and the outlet pipe  66  is fitted into a connection hole  67  in a side of the cylinder head  8  so that the inlet pipe  66  is communicated with the valve cam operation chamber  21 . 
     The valve cam operation chamber  21  thus communicated with the oil feed pipe  60  is communicated with a breather chamber  69  inside the side wall cap  30  via a gas-liquid separation passage  68  provided in the camshaft  26  and including a transverse hole  68   a  and a longitudinal hole  68   b , and the breather chamber  69  is communicated with the inside of the aforementioned air cleaner  4  via a breather pipe  70 . 
     As is clearly shown in FIGS. 4 and 9, a head cover  71  for blocking the open upper face of the valve cam operation chamber  21  is joined to the cylinder head  8  via a sealing member  72 . A suction chamber  74  communicated with the valve cam operation chamber  21  via a plurality of orifices  73 ,  73  . . . is formed in the head cover  71 . The suction chamber  74  has a flattened shape along the upper face of the valve cam operation chamber  21 , and is provided with four orifices  73 ,  73  . . . at four points in the bottom wall thereof. Long and short suction pipes  75  and  76  are formed integrally with the bottom wall of the suction chamber  74  in its central area, with a space between the long and short suction pipes  75  and  76  in the direction perpendicular to the axis of the camshaft  26 , so as to project inside the valve cam operation chamber  21 , and orifices  73  and  73  are provided in the suction pipes  75  and  76 . 
     As shown in FIGS. 6 to  8 , the suction, chamber  74  is communicated also with the inside of the oil tank  40  via an oil return pipe  78 . The oil return pipe  78  is formed integrally with the belt cover  36  along the edge thereof on the side opposite to that for the oil feed pipe  60 . An inlet pipe  79  projecting from the upper end of the oil return pipe  78  at the back of the belt cover  36  is formed integrally with the belt cover  36 , and the inlet pipe  79  is connected to an outlet pipe  80 , which is formed in the head cover  71 , via a connector  81 ,. so that the inlet pipe  79  is communicated with the suction chamber  74 . 
     Moreover, an outlet pipe  82  projecting from the lower end of the oil return pipe  78  at the back of the belt cover  36  is formed integrally with the belt cover  36 , and the outlet pipe  82  is fitted into a return hole  83  provided in the oil tank  40  so that the outlet pipe  82  is communicated with the inside of the oil tank  40 . The open end of the return hole  83  is positioned in the vicinity of the central part of the oil tank  40  so that the open end is above the liquid level of the oil inside the oil tank  40  regardless of the operational position of the engine E. 
     A driven member  84  driven by the above-mentioned recoil type starter  42  is fixed to the forward end of the crankshaft  13  which projects out of the oil tank  40 . 
     Oil mist is generated by the oil slinger  56  scattering the lubricating oil O inside the oil tank  40  due to rotation of the crankshaft  13  during operation of the engine E, and when the pressure of the crank chamber  23  decreases due to the ascending movement of the piston  15  the oil mist so generated is taken into the crank chamber  6   a  via the through hole  55  so lubricating the crankshaft  13  and the piston  15 . When the pressure of the crank chamber  6   a  increases due to the descending movement of the piston  15 , the one-way valve  61  opens and, as a result, the above-mentioned oil mist ascends inside the oil feed pipe  60  together with the blowby gas generated in the crank chamber  6   a  and is supplied to the valve cam operation chamber  21 , so lubricating the camshaft  26 , the cam followers  24  and  25 , etc. 
     When the oil mist and the blowby gas inside the valve cam operation chamber  21  enter the gas-liquid separation passage  68  inside the rotating camshaft  26 , gas and liquid are separated by centrifugation inside the passage  68 , the liquefied oil is returned to the valve cam operation chamber  21  via the transverse hole  68   a  of the gas-liquid separation passage  68 , but the blowby gas is taken into the engine E via the breather chamber  69 , the breather pipe  70  and the air cleaner  4 , in that order, during the intake stroke of the engine E. 
     Since the valve cam operation chamber  21  is communicated with the inside of the air cleaner  4  as aforementioned via the gas-liquid separation passage  68 , the breather chamber  69  and the breather pipe  70 , the pressure within the valve cam operation chamber  21  is maintained at or slightly below atmospheric pressure. 
     On the other hand, the pressure of the crank chamber  6   a  is negative on average since the positive pressure component alone of the pressure pulsations is discharged through the one-way valve  61 . The negative pressure is transmitted to the oil tank  40  via the through hole  55  and further to the suction chamber  74  via the oil return pipe  78 . The pressure in the suction chamber  74  is therefore lower than that of the valve cam operation chamber  21 , and the pressure in the oil tank  40  is lower than that in the suction chamber  74 . As a result, the pressure is transferred from the valve cam operation chamber  21  to the suction chamber  74  via the suction pipes  75  and  76  and the orifices  73 ,  73  . . . and further to the oil tank  40  via the oil return pipe  78 , and accompanying this transfer the oil mist inside the valve cam operation chamber  21  and the liquefied oil retained in the valve cam operation chamber  21  are drawn up into the suction chamber  74  through the suction pipes  75  and  76  and the orifices  73 ,  73  . . . and returned to the oil tank  40  through the oil return pipe  78 . 
     As mentioned above, since the four orifices  73 ,  73  . . . are provided at four points of the bottom wall of the suction chamber  74  and the orifices  73  and  73  are provided in the long and short suction pipes  74  and  75  projecting into the valve cam operation chamber  21  from the central part of the bottom wall with a space between the long and short suction pipes  74  and  75  in the directions perpendicular to the axis of the camshaft  26 , one of the six orifices  73 ,  73  . . . is immersed in the oil stored in the valve cam operation chamber  21  regardless of the operational position of the engine E such as an upright state (A), a leftward tilted state (B), a rightward tilted state (C), a leftward laid state (D), a rightward laid state (E) or an upside down state (F) as shown in FIG.  10  and the oil can be drawn up into the suction chamber  74 . 
     Since the oil mist so generated in the oil tank  40  is thus supplied to the crank chamber  6   a  and the valve cam operation chamber  21  of the OHC type four-cycle engine E utilising the pressure pulsations within the crank chamber  6   a  and the function of the one-way valve  61  and is returned to the oil tank  40 , the inside of the engine E can be lubricated reliably by the oil mist regardless of the operational position of the engine E; moreover a special oil pump for circulating the oil mist is unnecessary and the structure can thus be simplified. 
     Not only the oil tank  40  which is made of a synthetic resin but also the oil feed pipe  60  providing communication between the crank chamber  6   a  and the valve cam operation chamber  21  and the oil return pipe  78  providing communication between the suction chamber  74  and the oil tank  40  are placed outside the engine main body  1 , there is no obstacle to making the engine main body  1  thinner and more compact, and this can thus contribute greatly to a reduction in the weight of the engine E. In particular, since the externally placed oil feed pipe  60  and oil return pipe  78  are less influenced by heat from the engine main body  1 , overheating of the lubricating oil O can be prevented. Furthermore, the integral formation of the oil feed pipe  60 , the oil return pipe  78  and the belt cover  36  can contribute to a reduction in the number of parts and an enhancement in the assembly performance. 
     FIG. 11 shows a modified embodiment of the oil feed pipe  60  and the oil return pipe  78 , and in this case the oil feed pipe  60  and the oil return pipe  78  are formed from a tube which is made of a flexible material such as rubber and which is separated from the belt cover  36 . Since the other components are the same as those in the above-mentioned embodiment, the corresponding parts in the drawing are denoted by the same reference numerals and their explanation is omitted. 
     In accordance with the modified embodiment, the oil feed pipe  60  and the oil return pipe  78  can be freely fitted to connection points, wherever the points are located, by appropriately flexing the pipes  60  and  78 , and the degrees of freedom of the layout can be increased. 
     It is also possible in the above-mentioned first embodiment that a rotary valve operatively connected to the crankshaft  13  and operating so as to unblock the oil feed pipe  60  when the piston  15  descends, and to block the oil feed pipe  60  when the piston  15  ascends is provided instead of the one-way valve  61 . 
     Next, a second embodiment of the present invention is explained by reference to FIGS. 12 to  24 . 
     As shown in FIGS. 12 and 13, a carburettor  102  and an exhaust muffler  103  are attached to the back and front respectively of an engine main body  101  of a handheld type four-cycle engine E, and an air cleaner  104  is attached to the inlet of the carburettor  102 . A fuel tank  105  made of a synthetic resin is attached to the lower face of the engine main body  101 . The two ends of a crankshaft  113  project out through the engine main body  101  and an oil tank  140  adjacent to one side of the engine main body  101 , and a recoil type starter  142  which can be transmittably connected to a driven member  184  fixed to one end of the crankshaft  113  is mounted on the outer face of the oil tank  140 . 
     A cooling fan  143  that also functions as a flywheel is fixed to the other end of the crankshaft  113 . A plurality of fitting bosses  146  (one thereof is shown in FIG. 12) are formed on the outer face of the cooling fan  143 , and a centrifugal shoe  147  is pivotally supported on each of the fitting bosses  146 . These centrifugal shoes  147 , together with a clutch drum  148  fixed to a drive shaft  150  which will be described below, form a centrifugal clutch  149 , and when the rotational rate of the crankshaft  113  exceeds a predetermined value, the centrifugal shoes  147  are pressed onto the inner periphery of the clutch drum  148  due to the centrifugal force of the shoes so transmitting the output torque of the crankshaft  113  to the drive shaft  150 . The cooling fan  143  has a larger diameter than that of the centrifugal clutch  149 . 
     An engine cover  151  covering the engine main body  101  and its attachments excluding the fuel tank  140  is fixed at appropriate positions to the engine main body  101 , and a cooling air inlet  119  is provided between the engine cover  151  and the fuel tank  105 . External air is thus taken in via the cooling air inlet  119  by the cooling fan  143  rotating and supplied for cooling each part of the engine E. 
     A truncated cone shaped bearing holder  158  coaxially disposed with the crankshaft  113  is fixed to the engine cover  151 , and the bearing holder  158  supports the drive shaft  150  which rotates the cutter C of the trimmer T (see FIG. 1) via a bearing  159  in the same way as in the above-mentioned first embodiment. 
     Since the oil tank  140  and the starter  142  are disposed on one side and the cooling fan  143  and the centrifugal clutch  149  are disposed on the other side with the engine main body  101  placed therebetween, the weight balance of the engine E between the right and left is improved, and the centre of gravity of the engine E can be made closer to the central part of the engine main body  101  so enhancing the handling performance of the engine E. 
     Furthermore, since the cooling fan  143  having a larger diameter than that of the centrifugal shoes  147  is fixed to the crankshaft  113  between the engine main body  101  and the centrifugal clutch  149 , it is possible to avoid any increase in the size of the engine E due to the cooling fan  143 . 
     The structures of the engine main body  101  and the oil tank  140  are explained below by reference to FIGS. 12 to  15 ,  16 ,  20  and  21 . 
     In FIGS. 12 to  15 , the engine main body  101  includes a crankcase  106  having a crank chamber  106   a , a cylinder block  107  having one cylinder bore  107   a , and a cylinder head  108  having a combustion chamber  108   a  and intake and exhaust ports  109  and  110  which open into the combustion chamber  108   a , and a large number of cooling fins  138  are formed on the outer peripheries of the cylinder block  107  and the cylinder head  108 . 
     The crankshaft  113  housed in the crank chamber  106   a  is supported in the left and right side walls of the crankcase  106  via ball bearings  114  and  114 ′. In this case, the left-hand ball bearing  114  is equipped with a seal, and an oil seal  117  is provided so as to adjoin the outside of the right-hand ball bearing  114 ′. A piston  115  fitted in the cylinder bore  107   a  is conventionally connected to the crankshaft  113  via a connecting rod  116  in an ordinary manner. 
     The oil tank  140  is provided so as to be integrally formed with the left-hand wall of the crankcase  106 , and is arranged so that the end of the crankshaft  113  on the side of the sealed ball bearing  114  runs through the oil tank  140 . An oil seal  139  through which the crankshaft  113  runs is fitted in the outside wall of the oil tank  140 . 
     A belt guide tube  186  having a flattened cross-section is provided integrally with the roof of the oil tank  140 , the belt guide tube  186  running vertically through the roof of the oil tank  140  and having open upper and lower ends. The lower end of the belt guide tube  186  extends towards the vicinity of the crankshaft  113  inside the oil tank  140 , and the upper end is provided integrally with the cylinder head  108  so as to share a partition  185  with the cylinder head  108 . A line of circular sealing bead  187  is formed around the periphery of the upper end of the belt guide tube  186  and the cylinder head  108 , and the partition  185  projects above the sealing bead  187 . 
     As shown in FIGS. 16,  20  and  21 , a circular sealing groove  188   a  corresponding to the above-mentioned sealing bead  187  is formed in the lower end face of a head cover  136 , and a linear sealing groove  188   b  linking two sides of the circular groove  188   a  to each other is formed in the inner face of the cover  136 . A circular packing  189   a  is fitted in the circular sealing groove  188   a , and a linear packing  189   b  formed integrally with the circular packing  189   a  is fitted in the linear sealing groove  188   b . The head cover  136  is joined to the cylinder head  108  by means of a bolt  137  so that the sealing bead  187  and the partition  185  are pressed into contact with the circular packing  189   a  and the linear packing  189   b  respectively. 
     The belt guide tube  186  and one half of the head cover  136  form a first valve operation chamber  121   a , the cylinder head  108  and the other half of the head cover  136  form a second valve operation chamber  121   b , and the two valve operation chambers  121   a  and  121   b  are divided by the above-mentioned partition  185 . 
     Referring again to FIGS. 12 to  15 , the engine main body  101  and the oil tank  140  are divided into an upper block Ba and a lower block Bb on a plane which includes the axis of the crankshaft  113  and is perpendicular to the axis of the cylinder bore  107   a . That is, the upper block Ba integrally includes the upper half of the crankcase  106 , the cylinder block  107 , the cylinder head  108 , the upper half of the oil tank  140  and the belt guide tube  186 . The lower block Bb integrally includes the lower half of the crankcase  106  and the lower half of the oil tank  140 . These upper and lower blocks Ba and Bb are cast individually, and joined to each other by means of a plurality of bolts  112  (see FIG. 14) after each part has been machined. 
     An intake valve  118   i  and an exhaust valve  118   e  for opening and closing the intake port  109  and the exhaust port  110  respectively are provided in the cylinder head  108  so as to be parallel to the axis of the cylinder bore  107   a , and a spark plug  120  is screwed in so that the electrodes thereof are close to the central area of the combustion chamber  108   a.    
     A valve operation mechanism  122  for opening and closing the above-mentioned intake valve  118   i  and exhaust valve  118   e  is explained below by reference to FIGS. 13 to  17 . 
     The valve operation mechanism  122  includes a wrap-around type timing transmission  122   a  that runs from the inside of the oil tank  140  to the first valve operation chamber  121   a , and a cam system  122   b  that runs from the first valve operation chamber  121   a  to a second valve operation chamber  121   b.    
     The wrap-around type timing transmission  122   a  includes a drive pulley  123  fixed to the crankshaft  113  inside the oil tank  140 , a driven pulley  124  rotatably supported in the upper part of the belt guide tube  186 , and a timing belt  125  wrapped around these drive and driven pulleys  123  and  124 . On the side of the partition  185 , the end face of the driven pulley  124  is joined integrally to a cam  126  which forms a part of the cam system  122   b . The drive and driven pulleys  123  and  124  are toothed, and the drive pulley  123  drives the driven pulley  124  via the belt  125  at a reduction rate of ½. 
     A support wall  127  is formed integrally with the outside wall of the belt guide tube  186 , the support wall  127  rising inside the circular sealing bead  187  and being in contact with or in the vicinity of the inner face of the head cover  136 . A through hole  128   a  and a bottomed hole  128   b  arranged coaxially above the sealing bead  187  are provided in the support wall  127  and the partition  185  respectively. Both ends of a support shaft  129  are rotatably supported by the through hole  128   a  and the bottomed hole  128   b , and the above-mentioned driven pulley  124  and the cam  126  are rotatably supported on the middle part of the support shaft  129 . Before the head cover  136  is attached, the support shaft  129  is inserted from the through hole  128   a  into a shaft hole  135  of the driven pulley  124  and the cam  126 , and into the bottomed hole  126   b . After the insertion, the head cover  136  is joined to the cylinder head  108  and the belt guide tube  186 , so that the inner face of the head cover  136  sits opposite the outer end of the support shaft  129  so functioning as a stopper for preventing the shaft  129  from falling out of the through hole  128   a , and the bottom of the bottomed hole  128   b  restricts inward movement of the shaft  129 . The support shaft  129  is thus restricted in its inward and outward movement in the axial direction. 
     It is therefore unnecessary to provide a special stopper member for the support shaft  129 , the support shaft  129  can be lubricated inside the head cover  136 , oil leakage can be prevented by an oil-tight joint between the head cover  136  and the cylinder head  108 , and it is thus unnecessary to attach a special sealing member to the support shaft  129  so reducing the number of parts and the cost. Furthermore, the support wall  127  rising inside the sealing bead  187  has the through hole  128   a  at a higher position than that of the sealing bead  187 , the head cover  136  is formed so that the inner face of the head cover  136  is in contact with or in the vicinity of the outer face of the support wall  127 , and the head cover  136  can thus be made more compact while enabling the support shaft  129  to be detachable before attaching to the head cover  136 . 
     A pair of bearing bosses  130   i  and  130   e  projecting parallel to the support shaft  129  are formed integrally with the partition  185  on the side of the second valve operation chamber  121   b . The cam system  122   b  includes the above-mentioned cam  126 ; an intake rocker shaft  131   i  and an exhaust rocker shaft  131   e  rotatably supported in the above-mentioned bearing bosses  130   i  and  130   e  respectively; an intake cam follower  132   i  and an exhaust cam follower  132   e  fixed to one end of the rocker shafts  133   i  and  133   e  respectively inside the first valve operation chamber  121   a , the forward end of each of the intake cam follower  132   i  and the exhaust cam follower  132   e  being in sliding contact with the lower face of the cam  126 ; an intake rocker arm  133   i  and an exhaust rocker arm  133   e  fixed to the other end of the intake and exhaust rocker shafts  133   i  and  133   e  respectively inside the second valve operation chamber  121   b , the forward end of each of the intake rocker arm  133   i  and the exhaust rocker arm  133   e  being in contact with the upper end of each of the intake valve  118   i  and the exhaust valve  118   e ; and an intake spring  134   i  and an exhaust spring  134   e  mounted on the intake valve  118   i  and the exhaust valve  118   e  respectively and forcing them in the closing direction. 
     When the crankshaft  113  rotates, the drive pulley  123  rotating together with the crankshaft  113  rotates the driven pulley  124  and the cam  126  via the belt  125 , the cam  126  then rocks the intake and exhaust cam followers  132   i  and  132   e  with appropriate timing, the rocking movements are transmitted to the intake and exhaust rocker arms  133   i  and  133   e  via the corresponding rocker shafts  131   i  and  131   e , and the intake and exhaust rocker arms  133   i  and  133   e  so rocked can open and close the intake and exhaust valves  118   i  and  118   e  with appropriate timing while co-operatively working with the intake and exhaust springs  134   i  and  134   e.    
     In the timing transmission  122   a , since the driven pulley  124  and the cam  126  are rotatably supported by the support shaft  129  and the support shaft  129  is also rotatably supported in both side walls of the first valve operation chamber  121   a , the support shaft  129  rotates due to frictional drag during rotation of the driven pulley  124  and the cam  126 , the difference in rotational rate between the support shaft  129  and the driven pulley  124  and the cam  126  decreases and abrasion of the rotating and sliding areas can be suppressed. The durability of the cam  126  and the support shaft  129  can therefore be enhanced without employing any special material or surface treatment. 
     The cam  126  having a comparatively large diameter is placed on one side of the cylinder head  108  together with the driven pulley  124 , and only the intake and exhaust rocker arms  133   i  and  133   e  and the intake and exhaust rocker shafts  131   i  and  131   e  having a comparatively small diameter are placed immediately above the cylinder head  108 . The valve operation mechanism  122  therefore does not occupy a large volume above the cylinder head  108 , and it is possible to reduce the total height of the engine E thus making the engine E more compact. 
     Furthermore, the support shaft  129  and the intake and exhaust rocker shafts  131   i  and  131   e  are positioned at a higher position than that of the line of circular sealing bead  187  at the upper end of the cylinder head  108  and the belt guide tube  186 , it is therefore possible to assemble and disassemble the support shaft  129  and the intake and exhaust rocker shafts  131   i  and  131   e  above the sealing bead  187  without any obstruction therefrom in a state in which the head cover  136  is removed, and the ease of assembly and maintenance is extremely high. 
     The lubrication system of the above-mentioned engine E is explained below by reference to FIGS. 13 to  22 . 
     As shown in FIGS. 14 and 15, the oil tank  140  stores a predetermined amount of lubricating oil O poured in through an oil inlet  140   a . Inside the oil tank  140 , a pair of oil slingers  156   a  and  156   b  arranged on either side of the drive pulley  123  in the axial direction are press-fitted, etc. onto the crankshaft  113 . These oil slingers  156   a  and  156   b  extend in directions radially opposite to each other and the forward ends thereof are bent so as to move away from each other in the axial direction so that when the oil slingers  156   a  and  156   b  are rotated by the crankshaft  113 , at least one of the oil slingers  156   a  and  156   b  stirs and scatters the oil O stored inside the oil tank  140  so generating an oil mist regardless of the operational position of the engine E. In this case, the oil mist is sprinkled over a part of the timing transmission  122   a  which extends into the oil tank  140  from the first valve operation chamber  121   a , or the oil mist enters the first valve operation chamber  121   a , and the timing transmission  122   a  can thus be lubricated directly and this provides one lubrication system. 
     Another lubrication system includes, as shown in FIGS. 13 to  15  and  22 , a through hole  155  provided in the crankshaft  113  so as to provide communication between the inside of the oil tank  140  and the crank chamber  106   a ; an oil feed pipe  160  provided outside the engine main body  101  so as to connect the lower part of the crank chamber  106   a  to the lower part of the second valve operation chamber  121   b ; an oil recovery chamber  174  provided in the cylinder head  108  in order to draw up liquefied oil residing in the second valve operation chamber  121   b ; an oil return passage  178  formed between the cylinder head  108  and the oil tank  140  so as to provide communication between the oil recovery chamber  174  and the oil tank  140  via the first valve operation chamber  121   a ; and a one-way valve  161  provided in the lower part of the crank chamber  106   a  and allowing the flow of oil mist only in the direction from the crank chamber  106   a  to the oil feed pipe  160 . 
     An open end  155   a  of the above-mentioned through hole  155  inside the oil tank  140  is positioned in the central part or the vicinity thereof inside the tank  140  so that the open end  155   a  is always above the liquid level of the oil O inside the oil tank regardless of the operational position of the engine E. The drive pulley  123  and one of the oil slingers  156   a  are fixed to the crankshaft  113  with the open end  155   a  located therebetween so as not to block the open end  155   a.    
     The above-mentioned one-way valve  161  (see FIG. 13) includes a reed valve in the illustrated embodiment, closes when the pressure of the crank chamber  106   a  becomes negative accompanying the reciprocating motion of the piston  115  and opens when the pressure becomes positive. 
     The lower end of the oil feed pipe  160  is connected by fitting it onto a lower connection pipe  162   a  provided so as to project out of the outer face of the crankcase  106  (see FIG.  13 ), and the upper end of the oil feel pipe  160  is connected by fitting it onto an upper connection pipe  182   b  provided so as to project out of the outer face of the cylinder head  108  (see FIGS.  14  and  18 ). The inside of the upper connection pipe  182   b  is communicated with the lower part of the second valve operation chamber  121   b  on one side via a link passage  163  (see FIGS. 18 and 19) formed in the cylinder head  108  and having large dimensions and is communicated with the oil return passage  178  on the other side via a bypass  164  having orifices (see FIG.  18 ). 
     As shown in FIGS. 15,  20  and  21 , a partition plate  165  defining a breather chamber  169  in the upper part of the head cover  136  is fitted to the roof of the cover  136  by means of a plurality of support stays  166  and clips  167  fastened to the support stays  166 , the support stays  166  provided so as to project from the roof. The breather chamber  169  is communicated with the second valve operation chamber  121   b  on one side via a communication pipe  168  and a gap g between the inner face of the head cover  136  and the partition plate  165 , the communication pipe  168 , which has large dimensions, is formed integrally with the partition plate  165  and projects towards the second valve operation chamber  121   b . The breather chamber  169  is also communicated with the inside of the above-mentioned air cleaner  104  on the other side via a breather pipe  170 . In the breather chamber  169 , a mixture of oil and blowby gas is separated into gas and liquid, and a labyrinth wall  172  for promoting the gas-liquid separation is provided so as to project out of the inner face of the roof of the head cover  136 . 
     Welded to the partition plate  165  is a box-shaped partition  179  having one open face and T-shape when viewed from above, the box-shaped partition  179  forming the above-mentioned oil recovery chamber  174  in the space on the upper face of the partition plate  165 , and the oil recovery chamber  174  is therefore also T-shaped. 
     Two suction pipes  175  are formed integrally with the partition plate  165  so as to project therefrom, the two suction pipes  175  being communicated with the two ends respectively of the lateral bar of the T-shaped oil recovery chamber  174 . The forward end of each of the suction pipes  175  extends towards the vicinity of the base of the second valve operation chamber  121   b , and an opening in the tip of each of the suction pipes  175  forms an orifice  175   a.    
     Three suction pipes  176  are provided integrally with the upper wall of the partition plate  179  so as to project therefrom, the three suction pipes  176  being communicated with three positions corresponding to the tips of the lateral and longitudinal bars of the T-shape of the oil recovery chamber  174 . Each of the tips of these suction pipes  176  extends towards the vicinity of the roof of the breather chamber  169 , and an opening in the tip of each of the suction pipes  176  forms an orifice  176   a.    
     Furthermore, an orifice  180  is provided in the upper wall of the partition box  179 , the orifice  180  providing communication between an indentation  179   a  in the upper face of the partition box  179  and the oil recovery chamber  174 . 
     Moreover, one pipe  181  communicated with an area corresponding to the tip of the longitudinal bar of the T-shape of the oil recovery chamber  174  is provided integrally with the partition plate  165 . The tip of the pipe  181  is fitted into an inlet  178   a  of the above-mentioned oil return passage  178  via a grommet  182 , the inlet  178   a  opening onto the base of the second valve operation chamber  121   b . The oil recovery chamber  174  is thus connected to the oil return passage  178 . The above-mentioned pipe  181  is placed close to an inner face of the second valve operation chamber  121   b , and an orifice  181   a  for drawing up oil is provided in the area close to the above-mentioned inner face, the orifice  181   a  providing communication between the second valve operation chamber  121   b  and the pipe  181 . 
     Since the breather chamber  169  is communicated with the inside of the air cleaner  104  via the breather pipe  170 , the pressure of the breather chamber  169  is generally maintained at atmospheric pressure even during operation of the engine E, and the pressure of the second valve operation chamber  121   b  communicated with the breather chamber  169  via the communication pipe pipe  168  having a low flow resistance is generally the same as that of the breather chamber  169 . 
     Since the crank chamber  106   a  discharges only the positive pressure component of the pressure pulsations caused by the ascending and descending motion of the piston  115  into the oil feed pipe  160  through the one-way valve  161  during operation of the engine E, the pressure of the crank chamber  106   a  is negative on average, and since the second valve operation chamber  121   b  receiving the above-mentioned positive pressure is communicated with the breather chamber  169  via the communication pipe  168  having a small flow resistance, the pressure of the second valve operation chamber  121   b  is almost the same as that of the breather chamber  169 . Since the negative pressure of the crank chamber  106   a  is transmitted to the oil tank  140  via the through hole  155  of the crankshaft  113  and further to the oil recovery chamber  174  via the oil return passage  178 , the pressure of the oil recovery chamber  174  is lower than that of the second valve operation chamber  121   b  and the breather chamber  169 , and the pressures of the oil tank  140  and the first valve operation chamber  121   a  are lower than that of the oil recovery chamber  174 . 
     As shown in FIG. 22, when the pressure of the crank chamber  106   a  is denoted by Pc, the pressure of the oil tank  140  is denoted by Po, the pressure of the first valve operation chamber  121   a  is denoted by Pva, the pressure of the second valve operation chamber  121   b  is denoted by Pvb, the pressure of the oil recovery chamber  174  is denoted by Ps, and the pressure of the breather chamber  169  is denoted by Pb, the following relationship can therefore be satisfied. 
     
       
         
           Pvb=Pb&gt;Ps&gt;Po=Pva&gt;Pc 
         
       
     
     As a result, the pressures of the second valve operation chamber  121   b  and the breather chamber  169  are transferred to the oil recovery chamber  174  via the suction pipes  175  and  176  and the orifice  180 , further to the oil tank  140  via the oil return passage  178 , and then to the crank chamber  106   a.    
     Oil mist is generated by the oil slingers  156   a  and  156   b  stirring and scattering the lubricating oil O inside the oil tank  140  during operation of the engine E, the oil slingers  156   a  and  156   b  being rotated by the crankshaft  113 . As described above, the oil mist so generated is sprinkled over a part of the timing transmission  122   a  exposed inside the oil tank  140  from the belt guide tube  186 , that is, over the drive pulley  123  and part of the timing belt  125 , or the oil mist enters the first valve operation chamber  121   a , and the timing transmission  122   a  is thus lubricated directly. When the oil droplets are sprinkled over even a part of the timing transmission  122   a , the oil is transferred not only to the entire transmission  122   a  but also to the cam  126  due to operation of the timing transmission  122   a  so lubricating them effectively. 
     The oil mist generated in the oil tank  140  is drawn into the crank chamber  106   a  via the through hole  155  of the crankshaft  113  along the direction of the above-mentioned pressure flow so lubricating the area around the crankshaft  113  and the piston  115 . When the pressure of the crank chamber  106   a  becomes positive due to the piston  115  descending, the one-way valve  161  opens and the above-mentioned oil mist together with the blowby gas generated in the crank chamber  106   a  ascend through the oil feed pipe  160  and the link passage  163 , and are supplied to the second valve operation chamber  121   b  so lubricating each part of the cam system  122   b  inside the chamber  121   b , that is, the intake and exhaust rocker arms  133   i  and  133   e , etc. 
     In this case, a portion of the oil mist passing through the above-mentioned link passage  163  is shunted to the oil return passage  178  via the hole-shaped bypass  164 . It is therefore possible to control the amount of oil mist supplied to the second valve operation chamber  121   b  by setting the flow resistance of the bypass  164  appropriately. 
     The oil mist and the blowby gas inside the second valve operation chamber  121   b  are separated into gas and liquid by expansion and collision with the labyrinth wall  172  while being transferred to the breather chamber  169  through the communication pipe  168  and the gap g around the partition plate  165 , and the blowby gas is taken into the engine E via the breather pipe  170  and the air cleaner  104  in that order during the intake stroke of the engine E. 
     Since, when the engine E is in an upright state, the oil liquefied in the breather chamber  169  resides in the indentation  179   a  in the upper face of the partition box  179  or flows down the communication pipe  168  or through the gap g to reside on the base of the second valve operation chamber  121   b , the oil is drawn up into the oil recovery chamber  174  by means of the orifice  180  or the suction pipe  175  provided there. Since, when the engine E is in an upside down state, the above-mentioned liquefied oil resides on the roof of the head cover  136 , the oil is drawn up into the oil recovery chamber  174  by means of the suction pipe  176  provided there. 
     The oil thus drawn up into the oil recovery chamber  174  returns to the oil tank  140  via the pipe  181  and the oil return passage  178 . In this case, when the oil return passage  178  is communicated with the oil tank  140  via the first valve operation chamber  121   a  as in the illustrated embodiment, the oil discharged from the oil return passage  178  is sprinkled over the timing transmission  122   a , so conveniently lubricating it. 
     Since the above-mentioned breather chamber  169  is defined between the roof of the head cover  136  and the partition plate  165  attached to the inner wall of the head cover  136  and the above-mentioned oil recovery chamber  174  is defined between the upper face of the above-mentioned partition plate  165  and the partition box  179  welded to the partition plate  165 , the oil recovery chamber  174  and the breather chamber  169  can be provided in the head cover  136  without dividing the roof of the head cover  136 . Moreover, since the breather chamber  169  and the oil recovery chamber  174  are present inside the head cover  136 , even if some oil leaks from either of the chambers  169  and  174 , the oil simply returns to the second valve operation chamber  121   b  without causing any problems, it is unnecessary to check whether the two chambers  169  and  174  are oil tight and the production cost can thus be reduced. 
     Since the partition box  179  can be welded to the partition plate  165  before attaching the partition plate  165  to the head cover  136 , the oil recovery chamber  174  can easily be formed in the partition plate  165 . 
     Since the oil suction pipes  175  and  176  are formed integrally with the partition plate  165  and the partition box  179  respectively, the oil suction pipes  175  and  176  can easily be formed. 
     When the engine E is in an upside down state as shown in FIG. 23, the oil O stored in the oil tank  140  moves towards the roof of the tank  140 , that is, the side of the first valve operation chamber  121   a . Since the open end of the first valve operation chamber  121   a  inside the oil tank  140  is set so as to be at a higher level than the liquid level of the stored oil O by means of the belt guide tube  186 , the stored oil O is prevented from entering the second valve operation chamber  121   b , and it is possible to prevent excess oil from being supplied to the timing transmission  122   a  and maintain a predetermined amount of oil inside the oil tank  140  so allowing the oil slingers  156   a  and  156   b  to continuously generate oil mist. 
     When the engine E is laid on its side as shown in FIG. 24 during its operation, the stored oil O moves towards the side face of the oil tank  140 , however, since the open end of the first valve operation chamber  121   a  inside the oil tank  140  is set so as to be at a higher level than the liquid level of the stored oil O by means of the belt guide tube  186 , the stored oil O is prevented from entering the second valve operation chamber  121   b , and it is possible to prevent excess oil from being supplied to the timing transmission  122   a , and maintain a predetermined amount of oil inside the oil tank  140  so allowing the oil slingers  156   a  and  156   b  to continuously generate oil mist. 
     An oil droplet guide wall  190  (see FIGS. 15 and 24) is provided integrally with the oil tank  140  so as to project out from the inner wall of the oil tank  140 , the oil droplet guide wall  190  facing the upper side  125   a  of the timing belt  125  of the timing transmission  122   a  as it moves from the drive side to the driven side around the drive pulley  123 . 
     As a result, in the case where the engine E is laid on its side and the upper side  25   a  of the timing belt  125  substantially moves horizontally from the drive side to the driven side, even when the oil O stored inside the oil tank  140  is present beneath the timing belt  125 , a portion of the oil mist generated by the rotation of the oil slingers  156   a  and  156   b  attach to the oil droplet guide wall  190 , the oil aggregates to form oil droplets O′ which fall down onto the upper part of the timing belt  125  on the drive side, the oil droplets O′ are carried on the upper side  125   a  of the timing belt  125  to the side of the driven pulley  124  while hardly receiving any influence from the centrifugal force, and at the same time the oil droplets O′ move around to the back of the upper side  25   a  so lubricating the driven pulley  124  reliably. 
     In this case, if the oil droplet guide wall  190  is absent, most of the oil mist generated by the oil slingers  156   a  and  156   b  attaches to the lower side of the timing belt  125 , the oil droplets are detached from the timing belt  125  due to centrifugal force as the lower side of the timing belt  125  is driven around to the upper side by rotation of the drive pulley  123 , and it is difficult for the oil mist to reach the driven side of the timing belt  125 . 
     The lubrication system of the valve operation mechanism  122  can thus be divided into two parts, that is, a part for lubricating portions of the cam system  122   b  and the timing transmission  122   a  inside the first valve operation chamber  121   a  and the oil tank  140  with the oil scattered inside the oil tank  140 , and a part for lubricating the rest portions of the cam system  122   b  inside the second valve operation chamber  121   b  with the oil mist transferred to the second valve operation chamber  121   b . The burden put on each part of the lubrication system can thus be lessened and the entire valve operation mechanism  122  can be lubricated thoroughly. Moreover, each part of the engine E can be lubricated reliably by the use of oil droplets and oil mist regardless of the operational position of the engine E. 
     Since the oil mist generated inside the oil tank  140  is circulated by utilising the pressure pulsations inside the crank chamber  106   a  and the one-way transfer function of the one-way valve  161 , it is unnecessary to employ a special oil pump for circulating the oil mist and the structure can be simplified. 
     Not only the oil tank  140  but also the oil feed pipe  160  providing communication between the crank chamber  106   a  and the second valve operation chamber  121   b  are provided outside the engine main body  101 , and the weight of the engine E can therefore be greatly reduced without interfering with the engine main body  101  being made thinner and more compact. In particular, since the externally placed oil feed pipe  160  is hardly influenced by the heat of the engine main body  101  and easily releases its heat, cooling of the oil mist passing through the oil feed pipe  160  can be promoted. 
     Furthermore, since the oil tank  140  is placed on one exterior side of the engine main body  101 , the total height of the engine E can be greatly reduced, and since a part of the timing transmission  122   a  is housed inside the oil tank  140 , any increase in the width of the engine E can be minimised so making the engine E more compact. 
     Next, a third embodiment of the present invention is explained by reference to FIGS. 25 to  36 . 
     The external structure of the handheld type four-cycle engine E is explained by reference to FIGS. 25 and 26. 
     A carburettor  202  and an exhaust muffler  203  are attached to the front and back respectively of an engine main body  201  of the above-mentioned handheld type four-cycle engine E, and an air cleaner  204  is attached to the inlet of the carburettor  202 . A fuel tank  205  made of a synthetic resin is attached to the lower face of the engine main body  201 . The two ends of a crankshaft  213  project out of the engine main body  201  and an oil tank  240  adjacent to one side of the engine main body  201 , and a recoil type starter  242  which can be transmittably connected to a driven member  284  fixed to one end of the crankshaft  213  is attached to the outer face of the oil tank  240 . 
     A cooling fan  243  that also functions as a flywheel is fixed to the other end of the crankshaft  213 . A plurality of fitting bosses  246  (one thereof is shown in FIG. 25) are formed on the outer face of the cooling fan  243 , and a centrifugal shoe  247  is pivotally supported on each of the fitting bosses  246 . These centrifugal shoes  247 , together with a clutch drum  248  fixed to a drive shaft  250  which will be described below, form a centrifugal clutch  249 , and when the rotational rate of the crankshaft  213  exceeds a predetermined value, the centrifugal shoes  247  are pressed onto the inner periphery of the clutch drum  248  due to the centrifugal force of the shoes so transmitting the output torque of the crankshaft  213  to the drive shaft  250 . The cooling fan  243  has a larger diameter than that of the centrifugal clutch  249 . 
     An engine cover  251  covering the engine main body  201  and its attachments excluding the fuel tank  240  is fixed at appropriate positions to the engine main body  201 , and a cooling air inlet  219  is provided between the engine cover  251  and the fuel tank  205 . External air is thus taken in via the cooling air inlet  219  by the cooling fan  243  rotating and supplied for cooling each part of the engine E. 
     A truncated cone shaped bearing holder  258  coaxially arranged with the crankshaft  213  is fixed to the engine cover  251 , and the bearing holder  258  supports the drive shaft  250  which rotates the cutter C of the trimmer T (see FIG. 1) via a bearing  259  in the same way as in the above-mentioned first embodiment. 
     Since the oil tank  240  and the starter  242  are disposed on one side and the cooling fan  243  and the centrifugal clutch  249  are disposed on the other side with the engine main body  201  placed therebetween, the weight balance of the engine E between the right and left is improved, and the centre of gravity of the engine E can be made closer to the central part of the engine main body  201  so enhancing the handling performance of the engine E. 
     Furthermore, since the cooling fan  243  having a larger diameter than that of the centrifugal shoes  247  is fixed to the crankshaft  213  between the engine main body  201  and the centrifugal clutch  249 , it is possible to avoid any increase in the size of the engine E due to the cooling fan  243 . 
     The structures of the engine main body  201  and the oil tank  240  are explained below by reference to FIGS. 25 to  28 ,  29 ,  32  and  33 . 
     In FIGS. 25 to  28 , the engine main body  201  includes a crankcase  206  having a crank chamber  206   a , a cylinder block  207  having one cylinder bore  207   a , and a cylinder head  208  having a combustion chamber  208   a  and intake and exhaust ports  209  and  210  which open into the combustion chamber  208   a , and a large number of cooling fins  238  are formed on the outer peripheries of the cylinder block  207  and the cylinder head  208 . 
     The crankshaft  213  housed in the crank chamber  206   a  is supported in the left and right side walls of the crankcase  206  via ball bearings  214  and  214 ′. In this case, the left-hand ball bearing  214  is equipped with a seal, and an oil seal  217  is provided so as to adjoin the outside of the right-hand ball bearing  214 ′. A piston  215  fitted in the cylinder bore  207   a  is conventionally connected to the crankshaft  213  via a connecting rod  216  in an ordinary manner. 
     The oil tank  240  is provided so as to be integrally formed with the left-hand wall of the crankcase  206 , and is arranged so that the end of the crankshaft  213  on the side of the sealed ball bearing  214  runs through the oil tank  240 . An oil seal  239  through which the crankshaft  213  runs is fitted in the outside wall of the oil tank  240 . 
     A belt guide tube  286  having a flattened cross-section is provided integrally with the roof of the oil tank  240 , the belt guide tube  286  running vertically through the roof of the oil tank  240  and having open upper and lower ends. The lower end of the belt guide tube  286  extends towards the vicinity of the crankshaft  213  inside the oil tank  240 , and the upper end is provided integrally with the cylinder head  208  so as to share a partition  285  with the cylinder head  208 . A line of circular sealing bead  287  is formed around the periphery of the upper end of the belt guide tube  286  and the cylinder head  208 , and the partition  285  projects above the sealing bead  287 . 
     As shown in FIGS. 29,  32  and  33 , a circular sealing groove  288   a  corresponding to the above-mentioned sealing bead  287  is formed in the lower end face of a head cover  236 , and a linear sealing groove  288   b  linking two sides of the circular groove  288   a  to each other is formed in the inner face of the cover  236 . A circular packing  289   a  is fitted in the circular sealing groove  288   a , and a linear packing  289   b  formed integrally with the circular packing  289   a  is fitted in the linear sealing groove  288   b . The head cover  236  is joined to the cylinder head  208  by means of a bolt  237  so that the sealing bead  287  and the partition  285  are pressed in to contact with the circular packing  289   a  and the linear packing  289   b  respectively. 
     The belt guide tube  286  and one half of the head cover  236  form a first valve operation chamber  221   a , the cylinder head  208  and the other half of the head cover  236  form a second valve operation chamber  221   b , and the two valve operation chambers  221   a  and  221   b  are divided by the above-mentioned partition  285 . 
     Referring again to FIGS. 25 to  28 , the engine main body  201  and the oil tank  240  are divided into an upper block Ba and a lower block Bb on a plane which includes the axis of the crankshaft  213  and is perpendicular to the axis of the cylinder bore  207   a . That is, the upper block Ba integrally includes the upper half of the crankcase  206 , the cylinder block  207 , the cylinder head  208 , the upper half of the oil tank  240  and the belt guide tube  286 . The lower block Bb integrally includes the lower half of the crankcase  206  and the lower half of the oil tank  240 . These upper and lower blocks Ba and Bb are cast individually, and joined to each other by means of a plurality of bolts  212  (see FIG. 27) after each part has been machined. 
     An intake valve  218   i  and an exhaust valve  218   e  for opening and closing the intake port  209  and the exhaust port  210  respectively are provided in the cylinder head  208  so as to be parallel to the axis of the cylinder bore  207   a , and a spark plug  220  is screwed in so that the electrodes thereof are close to the central area of the combustion chamber  208   a.    
     A valve operation mechanism  222  for opening and closing the above-mentioned intake valve  218   i  and exhaust valve  218   e  is explained below by reference to FIGS. 26 to  30 . 
     The valve operation mechanism  222  includes a timing transmission  222   a  that runs from the inside of the oil tank  240  to the first valve operation chamber  221   a , and a cam system  222   b  that runs from the first valve operation chamber  221   a  to the second valve operation chamber  221   b.    
     The timing transmission  222   a  includes a drive pulley  223  fixed to the crankshaft  213  inside the oil tank  240 , a driven pulley  224  rotatably supported in the upper part of the belt guide tube  286 , and a timing belt  225  wrapped around these drive and driven pulleys  223  and  224 . On the side of the partition  285 , the end face of the driven pulley  224  is joined integrally to a cam  226  which forms a part of the cam system  222   b . The cam  226  is thus placed together with the driven pulley  114  on one side of the cylinder head  208 . The drive and driven pulleys  223  and  224  are toothed, and the drive pulley  223  drives the driven pulley  224  via the belt  225  at a reduction rate of ½. 
     A support wall  227  is formed integrally with the outside wall of the belt guide tube  286 , the support wall  227  rising inside the circular sealing bead  287  and being in contact with or in the vicinity of the inner face of the head cover  236 , and by means of a through hole  228   a  provided in the support wall  227  and a bottomed hole  228   b  provided in the partition  285 , both ends of a support shaft  229  are rotatably supported, and the above-mentioned driven pulley  236  and the cam  226  are rotatably supported on the middle part of the support shaft  229 . Before the head cover  236  is mounted, the support shaft  229  is inserted from the through hole  228   a  into a shaft hole  235  of the driven pulley  224  and the cam  226 , and into the bottomed hole  226   b . After the insertion, the head cover  236  is joined to the cylinder head  208  and the belt guide tube  286 , so that the inner face of the head cover  236  sits opposite the outer end of the support shaft  229  so functioning as a stopper for the support shaft  229 . 
     A pair of bearing bosses  230   i  and  230   e  projecting parallel to the support shaft  229  are formed integrally with the partition  285  on the side of second valve operation chamber  221   b . The cam system  222   b  includes the above-mentioned cam  226 ; an intake rocker shaft  231   i  and an exhaust rocker shaft  231   e  rotatably supported in the above-mentioned bearing bosses  230   i  and  230   e  respectively; an intake cam follower  232   i  and an exhaust cam follower  232   e  fixed to one end of the rocker shafts  233   i  and  233   e  respectively inside the first valve operation chamber  221   a , the forward end of each of the intake cam follower  232   i  and the exhaust cam follower  232   e  being in sliding contact with the lower face of the cam  226 ; an intake rocker arm  233   i  and an exhaust rocker arm  233   e  fixed to the other end of the intake and exhaust rocker shafts  233   i  and  233   e  respectively inside the second valve operation chamber  221   b , the forward end of each of the intake rocker arm  233   i  and the exhaust rocker arm  233   e  being in contact with the upper end of each of the intake valve  218   i  and the exhaust valve  218   e , and intake spring  234   i  and exhaust spring  234   e  mounted on the intake valve  218   i  and the exhaust valve  218   e  respectively and forcing them in the closing direction. 
     The support shaft  229  and the intake and exhaust rocker arms  231   i  and  231   e  are positioned above the circular sealing bead  287  on the upper ends of the cylinder head  208  and the belt guide tube  286 . 
     When the crankshaft  213  rotates, the drive pulley  223  rotating together with the crankshaft  213  rotates the driven pulley  224  and the cam  226  via the belt  225 , the cam  226  then rocks the intake and exhaust cam followers  232   i  and  232   e  with appropriate timing, the rocking movements are transmitted to the intake and exhaust rocker arms  233   i  and  233   e  via the corresponding rocker shafts  231   i  and  231   e , and the intake and exhaust rocker arms  233   i  and  233   e  so rocked can open and close the intake and exhaust valves  218   i  and  218   e  with appropriate timing while co-operatively working with the intake and exhaust springs  234   i  and  234   e.    
     In the timing transmission  222   a , since the driven pulley  224  and the cam  226  are rotatably supported by the support shaft  229  and the support shaft  229  is also rotatably supported in both side walls of the first valve operation chamber  221   a , the support shaft  229  rotates due to frictional drag during rotation of the driven pulley  224  and the cam  226 , the difference in rotational rate between the support shaft  229  and the driven pulley  224  and the cam  226  decreases and abrasion of the rotating and sliding areas can be suppressed so enhancing the durability. 
     The cam  226  having a comparatively large diameter is placed on one side of the cylinder head  208  together with the driven pulley  224 , and only the intake and exhaust rocker arms  233   i  and  233   e  and the intake and exhaust rocker shafts  231   i  and  231   e  having a comparatively small diameter are placed immediately above the cylinder head  208 . The valve operation mechanism  222  therefore does not occupy a large volume above the cylinder head  208 , and it is possible to reduce the total height of the engine E thus making the engine E more compact. 
     Furthermore, the support shaft  229  and the intake and exhaust rocker shafts  231   i  and  231   e  are positioned at a higher position than that of the line of circular sealing bead  287  at the upper end of the cylinder head  208  and the belt guide tube  286 , it is therefore possible to assemble and disassemble the support shaft  229  and the intake and exhaust rocker shafts  231   i  and  231   e  above the sealing bead  287  without any obstruction therefrom in a state in which the head cover  236  is removed, and the easy of assembly and maintenance is extremely high. 
     The lubrication system of the above-mentioned engine E is explained below by reference to FIGS. 26 to  34 . 
     As shown in FIGS. 27 and 28, the lubrication system of the engine E includes a first lubrication part La for lubricating the area around the crank shaft  213 , that is, the crank shaft  213 , the bearings  214  and  214 ′, the connecting rod  216 , the piston  215 , etc., and a second lubrication part Lb for lubricating the valve operation mechanism  222 . These parts La and Lb share the above-mentioned oil tank  240 . The oil tank  240  stores a predetermined amount of lubricating oil O poured in through an oil inlet  240   a . A pair of oil slingers  256   a  and  256   b  arranged on either side of the drive pulley  223  in the axial direction is press-fitted onto the crankshaft  213 . These oil slingers  256   a  and  256   b  extend in directions radially opposite to each other and the forward ends thereof are bent so as to move away from each other in the axial direction so that when the oil slingers  256   a  and  256   b  are rotated by the crankshaft  213 , at least one of the oil slingers  256   a  and  256   b  stirs and scatters the oil O stored inside the oil tank  240  so generating an oil mist regardless of the operational position of the engine E. 
     The first lubrication system La includes a first oil passage  260   1  provided through the crank shaft  213  and providing communication between the inside of the oil tank  240  and the crank chamber  206   a , and a second oil passage  260   2  providing communication between the base of the crank chamber  206   a  and the inside of the oil tank  240 , and a first one-way valve  261  is provided at the opening of the second oil passage  260   2  in the crank chamber  206   a . The first one-way valve  261  closes and opens as the pressure of the crank chamber  206   a  decreases and increases accompanying the ascent and descent respectively of the piston  215 . The first and second oil passages  260   1  and  260   2  are formed so that their open ends  260   1   a  and  260   2   a  inside the oil tank  240  are as close to the central part of the oil tank  240  as possible, with an arrangement such that the open ends  260   1   a  and  260   2   a  are always above the liquid level of the stored oil O regardless of the operational position of the engine E. 
     The second lubrication system Lb includes a third oil passage  260   3  provided through the engine main body  201  so as to provide communication between the middle part of the first valve operation chamber  221   a  and the base of the second valve operation chamber  221   b ; an oil recovery chamber  274  formed in the head cover  236  so as to be communicated with the second valve operation chamber  221   b ; a fourth oil passage  260   4  provided in the engine main body  201  so as to provide communication between the oil recovery chamber  274  and the crank chamber  206   a ; the second oil passage  260   2 ; and a second one-way valve  262  provided at the opening of the third oil passage  260   3  in the second valve operation chamber  221   b . The second one-way valve  262  closes and opens as the pressure of the crank chamber  206   a  decreases and increases accompanying the ascent and descent respectively of the piston  215 . 
     As shown in FIGS. 28,  32  and  33 , a partition plate  265  defining a breather chamber  269  in the upper part of the head cover  236  is fitted to the roof of the cover  236  by means of a plurality of support stays  266  and clips  267  fastened to the support stays  266 , the support stays  266  provided so as to project from the roof. The breather chamber  269  is communicated with the second valve operation chamber  221   b  on one side via a large gap, that is, a communication pipe  268  (see FIG. 32) between the periphery of the partition plate  264  and the inner face of the head cover  236 , and is communicated with the air cleaner  204  on the other side via a breather pipe  270 . The mixture of oil and blowby gas is separated into gas and liquid in the breather chamber  269 . 
     Welded to the partition plate  265  is a box-shaped partition  279  that forms the above-mentioned oil recovery chamber  274  in the space on the upper face of the partition plate  265 . 
     A plurality of suction pipes  275  (four in the illustrated embodiment) are provided integrally with the partition plate  265  so as to project therefrom, each of the suction pipes  275  being separated from the others and communicated with the oil recovery chamber  274 . The tip of each of the suction pipes  275  extends towards the vicinity of the base of the second valve operation chamber  221   b , and an opening at each of their tips forms an orifice  275   a.    
     A plurality of suction pipes  276  (three in the illustrated embodiment) are provided integrally with the upper wall of the partition plate  279  so as to project therefrom, each of the suction pipes  276  being separated from the others and communicated with the oil recovery chamber  274 . The tip of each of these suction pipes  276  extends towards the vicinity of the roof of the breather chamber  269 , and an opening at each of their tips forms an orifice  276   a.    
     Furthermore, orifices  280  and  283  are provided in the partition plate  265  and the partition box  279  respectively, the orifices  280  and  283  communicating the oil recovery chamber  274  with the second valve operation chamber  221   b and the breather chamber  269  respectively. 
     Moreover, one pipe  281  communicated with the oil recovery chamber  274  is provided integrally with the partition plate  265  so as to project therefrom. The tip of the pipe  281  is joined to the upper end of the fourth oil passage  260   4  via a grommet  282 , the upper end of the fourth oil passage  260   4  opening so as to project out of the base of the second valve operation chamber  221   b . The oil recovery chamber  274  is thus connected to the fourth oil passage  260   4 . 
     The action of the lubrication system of this engine E is explained below. Since the oil slingers  256   a  and  256   b  rotating together with the crankshaft  213  stir and scatter the oil O stored inside the oil tank  240  during operation of the engine E so generating an oil mist, the oil tank  240  and the first valve operation chamber  221   a  opening into the upper part of the oil tank  240  are filled with the oil mist. The timing transmission  222   a  housed inside the first valve operation chamber  221   a  is therefore lubricated directly with the oil mist. 
     The pressure pulsations in which the pressure repeatedly decreases and increases due to the ascent and descent of the piston  215  occur in the crank chamber  206   a . Accompanying the pressure pulsations, the oil mist generated in the oil tank  240  is transferred back and forth between the oil tank  240  and the crank chamber  206   a , and the oil mist introduced into the crank chamber  206   a  lubricates the area around the crankshaft  213 , that is, the crankshaft  213 , the bearings  214  and  214 ′, the connecting rod  216 , the piston  215 , etc. Since the first one-way valve  261  closes when the pressure of the crank chamber  206   a  decreases and opens when the pressure increases and a proportion of the positive component of the pressure pulsations is thus discharged into the oil tank  240  via the second oil passage  260   2 , when the oil mist liquefies in the crank chamber  206   a  and resides in the base of the chamber  206   a , the oil is pushed back to the oil tank  240  via the second oil passage  260   2  together with the above-mentioned positive pressure. 
     The pressure pulsations inside the crank chamber  206   a  also influence the second one-way valve  262  via the oil tank  240 , the first valve operation chamber  221   a  and the third oil passage  260   3 , and the second one-way valve  262  also closes when the pressure of the crank chamber  206   a  decreases and opens when the pressure increases. When the second one-way valve  262  opens, the oil mist inside the oil tank  240  is sequentially transferred to the first valve operation chamber  221   a , the third oil passage  260   3 , and the second valve operation chamber  221   b . In the meantime, the oil mist lubricates the timing transmission  222   a  in the first valve operation chamber  221   a , and the cam system  222   b  in the second valve operation chamber  221   b.    
     Since the positive pressure component of the pressure pulsations in the crank chamber  206   a  is discharged into the oil tank  240  via the first one-way valve  261 , and the pressure of the crank chamber  206   a  is negative on average, also the pressure of the oil recovery chamber  274  communicated with the crank chamber  206   a  via the fourth oil passage  260   4  becomes negative. On the other hand, since the breather chamber  269  is communicated with the inside of the air clear  204  via the breather pipe  270 , and the pressure of the breather chamber  269  is maintained generally at atmospheric pressure even during operation of the engine E, the pressure of the second valve operation chamber  221   b  communicated with the breather chamber  269  via communication pipe  268  is also generally at atmospheric pressure. As a result, the pressures of the second valve operation chamber  221   b  and the breather chamber  269  are transferred to the oil recovery chamber  274  via the suction pipes  275  and  276  and the orifices  280  and  283 , and the oil mist inside the second valve operation chamber  221   b  and the breather chamber  269  is also transferred to the oil recovery chamber  274  accompanying the move in pressure. In particular, while the engine E is in an upright state, the oil liquefied in the second valve operation chamber  221   b  and resides in the base thereof is transferred to the oil recovery chamber  274  by means of the oil suction pipe  275  positioned closely to the base of the second valve operation chamber  221   b . While the engine E is in an upside down state, the oil liquefied in the breather chamber  269  and residesd in the base thereof, that is, the roof of the head cover  236 , is transferred to the oil recovery chamber  274  by means of the oil suction pipe  276  positioned closely to the roof of the head cover  236 . The oil thus recovered in the oil recovery chamber  274  is transferred to the crank chamber  206   a  via the fourth oil passage  260   4  so lubricating the area around the crankshaft  213 . 
     The timing transmission  222   a  and the cam system  222   b  which have comparatively low load are lubricated with the oil mist alone that is introduced into the first and second valve operation chambers  221   a  and  221   b  from the oil tank  240 , the amount of lubricating oil is comparatively small and excessive lubrication can be avoided. The surroundings of the crankshaft  213  are lubricated with the oil mist that is introduced into the crank chamber  206   a  from the oil tank  240  and the oil mist, and the liquefied oil that are recovered in the oil recovery chamber  274  after lubrication of the cam system  222   b ; the amount of lubricating oil is comparatively large and it is possible to efficiently lubricate the area around the crankshaft  213  which suffers a comparatively high load. Since the surroundings of the crankshaft  213  and the valve operation mechanism  222  are lubricated according to their loading, the amount of circulating oil, that is, the amount of oil stored inside the oil tank  240  can be reduced in comparison with the conventional system, and the oil tank  240  and, therefore, the entire engine E can be made more compact and lighter. 
     Since the oil mist generated inside the oil tank  240  is circulated by utilising the pressure pulsations inside the crank chamber  206   a  and the one-way transfer functions of the first and second one-way valves  261  and  262 , it is unnecessary to employ a special oil pump for circulating the oil mist, and the structure can be simplified. 
     The blowby gas generated in the crank chamber  206   a  is transferred to the oil tank  240  via the first oil passage  260   1  and to the breather chamber  269  together with the oil mist via the first valve operation chamber  221   a , the third oil passage  260   3  and the second valve operation chamber  221   b , they are separated into gas and liquid in the breather chamber  269 , and the blowby gas separated from the oil is taken into the engine E via the breather pipe  270  and the air cleaner  204  in that order during the intake stroke of the engine E. 
     When the engine E is turned upside down as shown in FIG. 35 or laid on its side as shown in FIG. 36, the oil O stored in the oil tank  240  moves towards the roof or the side face of the oil tank  240 . However, since the open end of the first valve operation chamber  221   a  toward the oil tank  240  is set so as to always be at a higher level than the liquid level of the stored oil O by means of the belt guide tube  286 , the stored oil O is prevented from entering the first valve operation chamber  221   a , and it is possible to prevent excess oil from being supplied to the timing transmission  222   a , and maintain a predetermined amount of oil inside the oil tank  240  so allowing the oil slingers  256   a  and  256   b  to continuously generate an oil mist. 
     Since the oil tank  240  is placed on one exterior side of the engine main body  201 , the total height of the engine E can be greatly reduced, and since a part of the timing transmission  222   a  is housed inside the oil tank  240 , any increase in the width of the engine E can be minimised so making the engine E more compact and lighter. 
     In the above-mentioned third embodiment, the third oil passage  260   3  is connected to the middle part of the first valve operation chamber  221   a , but the third oil passage  260   3  may be connected to the upper part of the first valve operation chamber  221   a.    
     The present invention is not limited to the above-mentioned embodiments and can be modified in a variety of ways without departing from the spirit and scope of the invention. For example, each of the above-mentioned embodiments employs the belt type timing transmission, but a chain type timing transmission may be employed.