Patent Publication Number: US-9422835-B2

Title: Valve train system drive device for an internal combustion engine, and engine incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority under 35 USC 119 based on Japanese patent application No. 2014-026190, filed on Feb. 14, 2014. The entire subject matter of the referenced priority document, including specification claims and drawings, is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a valve train system drive device for an internal combustion engine, and to an engine incorporating the valve train system drive device. 
     2. Background Art 
     Conventionally, the concept of a valve train system drive mechanism, that is, a device that transmits the driving force of a crankshaft to an overhead-cam type of valve train system via an idle gear for an internal combustion engine, is known (refer e.g. to Japanese Patent Publication 2006-183623, also published as US pub. 2006/0137636). In the engine disclosed in Japanese Patent Publication 2006-183623, an idle shaft supports an idle gear thereon, and respective ends of the idle shaft are supported by a boss part on a side surface of a crankcase, and by a boss part on a side surface of a crankcase cover. In addition, the idle shaft is fastened to a holder member fastened and fixed to the crankcase cover, and is thereby prevented from rotating. 
     However, in the above-described known valve train system drive mechanism for an internal combustion engine, a separate holder member is required on the crankcase cover in order to support the idle shaft. Therefore, this known system has problems in respect to size reduction, weight reduction, and simplification of the engine. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above-described background circumstances, and it is an object of the present invention to provide a valve train system drive mechanism for an internal combustion engine allowing for size reduction, weight reduction, and simplification of the engine. 
     Reference numbers are included in the following description, corresponding to the reference numbers used in the drawings. Such reference numbers are provided for illustration and are not intended to limit the invention. 
     To achieve the above object, the present invention provides a valve train system drive mechanism for an internal combustion engine that is provided in an overhead-cam type engine ( 1 ) having a crankshaft ( 10 ) provided in a crankcase ( 11 ) and dual valve train systems ( 45   f ,  45   r ). The engine has an idle gear ( 62 ) that transmits power received from the crankshaft ( 10 ) to the valve train systems ( 45   f ,  45   r ), an idle shaft ( 63 ) that is provided in parallel to the crankshaft ( 10 ) and which rotatably supports the idle gear ( 62 ), and a case cover ( 39 ) that covers the crankcase ( 11 ) from a lateral side. 
     A first aspect of the present invention is characterized by having the following configuration in this valve train system drive device. A crankcase-side boss part ( 90 ) is formed on a wall ( 20   b ) of the crankcase ( 11 ) and supports one end ( 63   b ) of the idle shaft ( 63 ), and a case-cover-side boss part ( 91 ,  291 ) is formed inside the case cover ( 39 ) and supports another end ( 63   c ) of the idle shaft ( 63 ). Furthermore, joint parts ( 128 ,  111 ), that engage with each other are formed at the case-cover-side boss part ( 91 ,  291 ) and the other end ( 63   c ) of the idle shaft ( 63 ), and engagement of the joint parts ( 128 ,  111 ) with each other precludes the idle shaft ( 63 ) from rotating. 
     According to the first aspect of the present invention, the one end and the other end of the idle shaft are supported by the crankcase-side boss part formed on the wall of the crankcase and by the case-cover-side boss part formed inside the case cover. In addition, the idle shaft can be made incapable of rotating by the joint parts at the other end of the idle shaft. Thus, the structure to support the idle shaft can be simplified. This allows for size reduction, weight reduction, and simplification of the engine. 
     Furthermore, a second aspect of the present invention is characterized by the following configuration. The idle gear ( 62 ) is rotatably supported by the idle shaft ( 63 ) with the bearing members ( 92   a ,  92   b ) interposed therebetween, and an idle shaft oil passage ( 112 ) is formed in the idle shaft ( 63 ). The idle shaft oil passage ( 112 ) is fluidly connected to an oil supply passage ( 58 ) formed in the wall ( 20   b ) of the crankcase ( 11 ) at the one end ( 63   b ) of the idle shaft ( 63 ). Furthermore, the case-cover-side boss part ( 91 ,  291 ) has a first abutting surface ( 123 ) that abuts against the bearing member ( 92   a ) in an axial direction, and an oil reservoir ( 124 ) having a concave shape is provided at an outer periphery of the first abutting surface ( 123 ). 
     According to the second aspect of the present invention, oil can be supplied to the bearing members from the oil reservoir on the outer peripheral side of the first abutting surface of the case-cover-side boss part, and therefore, the bearing members can be fed with oil by a simple structure. 
     In addition, a third aspect of the present invention is characterized in that a notch ( 125 ,  225 ) is made in the first abutting surface ( 123 ) to allow the first abutting surface ( 123 ) to communicate with the oil reservoir ( 124 ), and the idle shaft oil passage ( 112 ) communicates with the oil reservoir ( 124 ) via the notch ( 125 ,  225 ). 
     According to the third aspect of the present invention, the oil in the oil reservoir can be sufficiently supplied to the bearing members via the notch. 
     Moreover, a fourth aspect of the present invention is characterized in that a second abutting surface ( 122 ) is provided on the case-cover-side boss part at an outer periphery of the oil reservoir ( 124 ), and the second abutting surface ( 122 ) protrudes in the axial direction relative to the first abutting surface ( 123 ). 
     According to the fourth aspect of the present invention, the capacity of the oil reservoir can be set large, which allows effective oil feed to the bearing members. 
     Furthermore, a fifth aspect of the present invention is characterized in that a thrust bearing ( 94 ) is interposed between the second abutting surface ( 122 ) and the idle gear ( 62 ). 
     According to the fifth aspect of the present invention, the thrust bearing can be fed with oil from the oil reservoir. 
     Furthermore, a sixth aspect of the present invention is characterized in that a second notch ( 126 ,  226 ) to allow the oil reservoir ( 124 ) to communicate with a space outside the case-cover-side boss part ( 91 ,  291 ) is made in the second abutting surface ( 122 ). 
     According to the sixth aspect of the present invention, when excess oil is supplied to the oil reservoir, the oil can be discharged to the space outside the case-cover-side boss part. 
     In addition, a seventh aspect of the present invention is characterized in that a first notch ( 125 ,  225 ) is made in the first abutting surface ( 123 ) to allow the first abutting surface ( 123 ) to communicate with the oil reservoir ( 124 ), and the first notch ( 125 ,  225 ) and the second notch ( 126 ,  226 ) are made at positions different from each other in a circumferential direction of the case-cover-side boss part ( 91 ,  291 ). 
     According to the seventh aspect of the present invention, oil can be evenly supplied to the oil reservoir and the capacity of the oil reservoir can be efficiently used. 
     Moreover, an eighth aspect of the present invention is characterized by the following configuration. The first notches ( 125 ) are made as a pair of notches with placement in which the first notches ( 125 ) are opposed to each other in a radial direction on the ring-shaped abutting surface ( 123 ), and the second notches ( 126 ) are made as a pair of notches with placement opposed to each other on the second ring-shaped abutting surface ( 122 ). Furthermore, a straight line (L 1 ) linking the first notches ( 125 ) is substantially orthogonal to a straight line (L 2 ) linking the second notches ( 126 ). 
     According to the eighth aspect of the present invention, oil can be evenly supplied to the oil reservoir, and the capacity of the oil reservoir can be efficiently used. 
     Furthermore, a ninth aspect of the present invention is characterized in that an idle shaft oil passage ( 112 ) is formed in the idle shaft ( 63 ), and the oil passage ( 112 ) is connected to an oil passage ( 58 ) formed in the wall ( 20   b ) of the crankcase ( 11 ) at the one end ( 63   b ) of the idle shaft ( 63 ). In addition, in the ninth aspect hereof, an elastic spring member ( 95 ) is interposed between the other end ( 63   c ) of the idle shaft ( 63 ) and the case-cover-side boss part ( 91 ,  291 ). 
     According to the ninth aspect of the present invention, even when variation is caused in an oil pressure of oil passing through the oil passage in the idle shaft, the idle shaft can be supported in the axial direction by the elastic spring member, and generation of sounds attributed to the vibration of the idle shaft can be reduced or prevented. 
     Effects of the Invention 
     With the valve train system drive mechanism for an internal combustion engine according to the present invention, size reduction, weight reduction, and simplification of the engine can be achieved. 
     Furthermore, the bearing members can be fed with oil by a simple structure. 
     In addition, the oil in the oil reservoir can be sufficiently supplied to the bearing members via the notch. 
     Moreover, the capacity of the oil reservoir can be set large, which allows effective oil feed to the bearing members. 
     Furthermore, the thrust bearing can be fed with oil from the oil reservoir. 
     In addition, when excess oil is supplied to the oil reservoir, the oil can be discharged to the space outside the case-cover-side boss part. 
     Moreover, oil can be evenly supplied to the oil reservoir and the capacity of the oil reservoir can be efficiently used. 
     Furthermore, generation of sounds attributed to vibration of the idle shaft can be reduced or prevented. 
     For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side plan view of an engine, partially shown in cross-section and including a valve train system drive device of the present invention. 
         FIG. 2  is a cross-sectional view of the engine of  FIG. 1 , obtained by cutting the engine along a plane traversing the engine along the axial direction of a crankshaft. 
         FIG. 3  is a side view showing a timing chain and gear structure around valve train mechanisms. 
         FIG. 4  is a perspective view of a part around a cover member as viewed from the outside. 
         FIG. 5  is a diagram of the cover member as viewed from the inside. 
         FIG. 6  is a cross-sectional detail view of a part of the engine around an idle shaft supporting an idle gear thereon. 
         FIG. 7  is a perspective detail view of the part of the engine around the idle gear as viewed from a lateral side. 
         FIG. 8  is a top plan view of the idle gear. 
         FIG. 9  is a perspective detail view showing a part of the engine around a crankcase-side boss part. 
         FIG. 10  is a perspective detail view similar to  FIG. 9 , showing a state in which an idle shaft is attached to the crankcase-side boss part. 
         FIG. 11  is a plan view of a case-cover-side boss part; and 
         FIG. 12  is a plan view of a case-cover-side boss part, in a modified example according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     An internal combustion engine including an improved valve train system drive mechanism, according to a first illustrative embodiment of the present invention, will be described below with reference to the drawings. Throughout the following description, relative terms like “upper”, “lower”, “above”, “below”, “front”, “back”, and the like are used in reference to a vantage point of an operator of the vehicle, seated on the driver&#39;s seat and facing forward. It should be understood that these terms are used for purposes of illustration, and are not intended to limit the invention. 
       FIG. 1  is a side plan view of an engine, partially shown in cross-section and including a valve train system drive device of the present invention. 
     As shown in  FIG. 1 , in the depicted embodiment, the engine  1  is a four-cylinder V-type quad overhead cam engine configured to be mounted in a motorcycle (not shown). The engine  1  includes a crankcase  11  in which a crankshaft  10  is housed, a first bank  12   f  that tilts forward and extends forward and upward from the upper part of the crankcase  11 , and a second bank  12   r  that tilts rearward and extends rearward and upward from the upper part of the crankcase  11 . The first bank  12   f  and the second bank  12   r  respectively include cylinder blocks  13   f  and  13   r , cylinder heads  14   f  and  14   r  joined to the upper surfaces of the cylinder blocks  13   f  and  13   r , and head covers  15   f  and  15   r  that cover the upper surfaces of the cylinder heads  14   f  and  14   r.    
     The crankcase  11  is formed with a vertically-split structure and includes an upper case  11   a  and a lower case  11   b  joined to the lower surface of the upper case  11   a . The cylinder blocks  13   f  and  13   r  are formed monolithically with the upper case  11   a.    
     An oil pan  40  for storing and circulating oil is provided on the lower surface of the lower case  11   b . An oil cooler  41  is provided on the front surface of the lower case  11   b.    
     The engine  1  is configured to be mounted transversely in the motorcycle in an orientation such that the crankshaft  10  is oriented along the vehicle width direction, and the oil cooler  41  is located on the front surface of the engine  1 . 
       FIG. 2  is a sectional view obtained by cutting the engine  1  by a plane traversing the engine  1  along the axial direction of the crankshaft  10 . The first bank  12   f  and the second bank  12   r  are similarly formed. Therefore, in  FIG. 2 , a section of the first bank  12   f  is shown and corresponding diagrammatic representation of a section of the second bank  12   r  is omitted. 
     As shown in  FIGS. 1 and 2 , the crankshaft  10  is housed in a crank chamber  16  at the front part of the crankcase  11 , and a pair of cylinder bores  17  are made in each of the cylinder blocks  13   f  and  13   r  over the crank chamber  16 . Pistons  18  are provided in the cylinder bores  17  and are joined to the crankshaft  10  via connecting rods  19 . 
     The crankshaft  10  is supported by crank support parts  21  made in left and right sidewalls  20   a  and  20   b  of the crankcase  11  and a crank support part  22  made in an internal wall  20   c  in the crank chamber  16 . The crank support parts  21  and  22  are made at the connecting part between the upper case  11   a  and the lower case  11   b.    
     The crankshaft  10  has, at one end, a protrusion part  23  protruding outward from the sidewall  20   a  on the left side (one side), and a generator  24  is provided at the protrusion part  23 . The generator  24  is covered by a generator cover  25  attached to the sidewall  20   a.    
     The crankshaft  10  has, at the other end, a protrusion part  26  protruding outward from the sidewall  20   b  (wall of the crankcase) on the right side (the other side), and a primary drive gear  27  is provided at the protrusion part  26 . 
     A transmission chamber  28  is set at the rear part of the crankcase  11  and a constant mesh gear transmission  29  is housed in the transmission chamber  28 . The transmission chamber  28  and the crank chamber  16  are compartmentalized in the front-rear direction by a partition wall  30 . 
     The gear transmission  29  includes a main shaft  31  provided in parallel to the crankshaft  10 , a countershaft  32  provided in parallel to the main shaft  31 , and a gear train  33  provided between the main shaft  31  and the countershaft  32 . The countershaft  32  has an end part protruding outward from the sidewall  20   a . At this end part, a drive sprocket  34  that drives a driven sprocket of a rear wheel through a chain is provided. 
     The main shaft  31  includes a clutch support part  35  protruding outward from the sidewall  20   b  and a clutch system  36  is provided at the clutch support part  35 . The clutch system  36  is a well-known one including a clutch outer  36   a  provided on the clutch support part  35  rotatably relative to the main shaft  31 , a clutch inner  36   b  fixed to the main shaft  31 , a friction plate  36   c  provided between the clutch inner  36   b  and the clutch outer  36   a , and a clutch spring  37   d . To the clutch outer  36   a , a primary driven gear  38  meshing with the primary drive gear  27  is fixed. 
     The clutch system  36  and the primary drive gear  27  are covered by a clutch cover  39  (case cover) attached to a side surface of the crankcase  11 . 
     Over the primary drive gear  27 , a pair of cam chain chambers (timing chambers)  43   f  and  43   r  extending in the upward-downward direction in the engine  1  along the sidewall  20   b  to the sides of the head covers  15   f  and  15   r  are formed. The cam chain chambers  43   f  and  43   r  unite with each other at the lower parts and form one chamber near the protrusion part  26 . 
     Valve train mechanisms  45   f  and  45   r  (valve train systems) of the overhead-camshaft type are provided at the upper parts of the cylinder heads  14   f  and  14   r , respectively. The valve train mechanisms  45   f  and  45   r  are formed similarly between the first bank  12   f  and the second bank  12   r . Therefore, the valve train mechanism  45   f  of the first bank  12   f  will be mainly described herein, and corresponding components formed in the same manner as the valve train mechanism  45   f  in the valve train mechanism  45   r  are given the same symbols. 
     The valve train mechanism  45   f  includes intake valves  46 , exhaust valves  47 , valve springs  48  that bias the intake valves  46  and the exhaust valves  47  in the valve-closing direction, valve lifters  51  that press the intake valves  46  and the exhaust valves  47  in the valve-opening direction, an intake camshaft  49  provided on the intake side, and an exhaust camshaft  50  provided on the exhaust side. For each cylinder, a pair of intake valves  46  and a pair of exhaust valves  47  are provided. 
     The intake camshaft  49  has cam lobes  49   a  provided with predetermined height and phase. The cam lobes  49   a  selectively press the intake valves  46  through the valve lifters  51  in association with the rotation of the intake camshaft  49 . Therefore, the intake valves  46  move and intake ports  55  of the cylinder heads  14   f  and  14   r  are selectively opened and closed. 
     The exhaust camshaft  50  has cam lobes (not shown) provided with predetermined height and phase. The cam lobes selectively press the exhaust valves  47  through the valve lifters  51  in association with the rotation of the exhaust camshaft  50 . Therefore, the exhaust valves  47  move and exhaust ports  56  of the cylinder heads  14   f  and  14   r  are selectively opened and closed. 
       FIG. 3  is a side view showing timing chain and gear structure around valve train mechanisms. 
     Referring to  FIGS. 1 to 3 , the intake camshaft  49  and the exhaust camshaft  50  are provided in parallel to the crankshaft  10 . 
     In the first bank  12   f , the intake camshaft  49  has an intake-side driven sprocket  52  at the part protruding into the cam chain chamber  43   f . The exhaust camshaft  50  has an exhaust-side driven sprocket  53  at the part protruding into the cam chain chamber  43   f.    
     In the second bank  12   r , the intake camshaft  49  has the intake-side driven sprocket  52  at the part protruding into the cam chain chamber  43   r . The exhaust camshaft  50  has the exhaust-side driven sprocket  53  at the part protruding into the cam chain chamber  43   r.    
     The valve train mechanisms  45   f  and  45   r  are driven by a valve train system drive device  60  provided in the engine  1 . 
     The valve train system drive device  60  includes the following components: an idler drive gear  61  provided at the protrusion part  26  of the crankshaft  10 ; an idle gear  62  meshing with the idler drive gear  61 ; an idle shaft  63  that rotatably supports the idle gear  62 ; a first-bank-side cam chain (timing chain)  64   f  that transmits the rotation of the idle gear  62  to the intake-side driven sprocket  52  and the exhaust-side driven sprocket  53  in the first bank  12   f ; and a second-bank-side cam chain (timing chain)  64   r  that transmits the rotation of the idle gear  62  to the intake-side driven sprocket  52  and the exhaust-side driven sprocket  53  in the second bank  12   r.    
     The first bank  12   f  and the second bank  12   r  are so disposed as to be offset from each other in the axial direction of the crankshaft  10 . Corresponding to this, the first-bank-side cam chain  64   f  and the second-bank-side cam chain  64   r  are also so disposed as to be offset from each other in the vehicle width direction. 
     The idler drive gear  61  is formed with a smaller diameter than the primary drive gear  27  and is disposed on the shaft end side of the crankshaft  10  relative to the primary drive gear  27 . 
     The idle gear  62  is rotated in a rotational direction R in  FIG. 3  through the idler drive gear  61 . 
     In the cam chain chamber  43   f , a chain guide  65   f  in contact with the outer circumference of the first-bank-side cam chain  64   f  on the tight side and a chain tensioner  66   f  in contact with the outer circumference of the first-bank-side cam chain  64   f  on the loose side are provided. 
     In the cam chain chamber  43   r , a chain guide  65   r  in contact with the outer circumference of the second-bank-side cam chain  64   r  on the tight side and a chain tensioner  66   r  in contact with the outer circumference of the second-bank-side cam chain  64   r  on the loose side are provided. 
     The lower end parts of the chain guides  65   f  and  65   r  and the chain tensioners  66   f  and  66   r  are located closer to the sidewall  20   b  than the idle gear  62  near the crankshaft  10  and overlap with the idle gear  62  in side view. 
     Furthermore, in the cam chain chamber  43   f , a tensioner lifter  67   f  that biases the chain tensioner  66   f  toward the first-bank-side cam chain  64   f  is provided. In the cam chain chamber  43   r , a tensioner lifter  67   r  that biases the chain tensioner  66   r  toward the second-bank-side cam chain  64   r  is provided. 
     As shown in  FIG. 1 , at the lower part of the crankcase  11 , an oil pump  70  driven by power of the crankshaft  10  is provided. An oil strainer  57  extending to the bottom part of the oil pan  40  is connected to the oil pump  70  and the oil pump  70  sends oil sucked from the oil strainer  57  to the respective parts of the engine  1 . 
     The oil discharged from the oil pump  70  passes through an oil passage  71  at the front part of the crankcase  11  and reaches an oil filter  72 . Then, after passing through the oil filter  72  to be purified, the oil flows into the oil cooler  41  to be cooled. The oil that has passed through the oil cooler  41  flows into a main gallery  73  extending in substantially parallel to the crankshaft  10  below the crankshaft  10  and flows from the main gallery  73  to the respective lubrication points through branching. 
     Part of the oil branched from the main gallery  73  passes through an oil passage  74  in the partition wall  30  and is supplied to the gear transmission  29 . Furthermore, part of the oil branched from the main gallery  73  is sent from plural oil passages  75  made in the crankcase  11  to the upper side and the crank support parts  21  and  22  are lubricated with the oil. 
     Part of the oil that has reached the crank support parts  21  and  22  flows into an upper oil passage  76  made in substantially parallel to the main gallery  73  at the upper part of the crankcase  11  and part of the oil in the upper oil passage  76  is injected toward the pistons  18 . Moreover, part of the oil in the upper oil passage  76  passes through oil passages  77  running in the upward-downward direction in the first bank  12   f  and the second bank  12   r  and is supplied to the valve train mechanisms  45   f  and  45   r.    
     Furthermore, part of the oil branched from the main gallery  73  passes through an oil passage  85  ( FIG. 5 ) and reaches an oil chamber  78  ( FIG. 2 ) set inside the clutch cover  39 . Specifically, the oil chamber  78  has a cylindrical part  78   a  provided on the clutch cover  39  and a sealing member  78   b  that closes the end of the cylindrical part  78   a . The oil in the oil chamber  78  passes through a pipe  79  that penetrates the sealing member  78   b  and is connected to a shaft end of the crankshaft  10 , and reaches a shaft oil passage  80  in the crankshaft  10 . The oil in the shaft oil passage  80  is supplied to the joint parts between the crankshaft  10  and the connecting rods  19 , and so forth. 
       FIG. 4  is a perspective view of the part around the clutch cover  39  as viewed from the outside.  FIG. 5  is a diagram of the clutch cover  39  as viewed from the inside. In  FIG. 4 , a state in which the cylinder heads  14   f  and  14   r  are removed is shown. 
     As shown in  FIGS. 2, 4, and 5 , the clutch cover  39  monolithically has a side cover part  81  that has a substantially flat plate shape and covers the protrusion part  26  of the crankshaft  10 , the idler drive gear  61 , and so forth from the lateral outside and a clutch cover part  82  that covers the clutch system  36  from the outside on the rear side of the side cover part  81 . 
     The side cover part  81  covers the range from the upper part of the crankcase  11  near the cylinder blocks  13   f  and  13   r  to the lower part of the crankcase  11 . The clutch cover part  82  is formed into a bottomed cylindrical shape along the clutch system  36  and bulges toward the lateral outside relative to the side cover part  81 . 
     The clutch cover  39  has plural fixing holes  39   a  at the peripheral part and is fixed to a side surface of the crankcase  11  by cover fixing bolts (not shown) inserted into the fixing holes  39   a . In the side surface of the crankcase  11 , plural fixing holes  11   c  into which the cover fixing bolts are fastened are made. 
     ***On the outer surface of the side cover part  81 , a pipe-shaped part  83  extending in the front-rear direction and a pipe-shaped part  84  extending upward from the rear end of the pipe-shaped part  83  are formed. The oil passage  85  is formed inside the pipe-shaped part  83  and the pipe-shaped part  84 . The oil passage  85  is connected to the main gallery  73  and is connected to the upper oil passage  76  via the upper end part of the pipe-shaped part  84 . Furthermore, the oil passage  85  communicates with the oil chamber  78  at the rear end part of the pipe-shaped part  83  and part of the oil in the main gallery  73  passes through the oil passage  85  to be supplied to the oil chamber  78 . 
       FIG. 6  is a sectional view of the part around the idle gear  62 .  FIG. 7  is a perspective view of the part around the idle gear  62  as viewed from the lateral side.  FIG. 8  is a plan view of the idle gear  62 . 
     Referring to  FIGS. 2, 3, and 6 to 8 , both ends of the idle shaft  63  are supported by a crankcase-side boss part  90  provided on the sidewall  20   b  of the crankcase  11  and a case-cover-side boss part  91  provided on the clutch cover  39 . 
     The idle gear  62  is rotatably journaled by the idle shaft  63  with the intermediary of a pair of bearings  92   a  and  92   b  (bearing members) fitted to outer circumference  63   a  of the idle shaft  63 . 
     For example, the bearings  92   a  and  92   b  are roller bearings each having a cylindrical case and plural rollers held on the outer circumferential part of this case. 
     A case-side thrust bearing  93  having a ring shape is interposed between one end of the idle gear  62  and the crankcase-side boss part  90 . Furthermore, a cover-side thrust bearing  94  (thrust bearing) having a ring shape is interposed between the other end of the idle gear  62  and the case-cover-side boss part  91 . Moreover, a coil-shaped spring  95  (elastic spring member) that biases the idle shaft  63  in the axial direction is provided between the idle shaft  63  and the case-cover-side boss part  91 . 
     The idle gear  62  monolithically has the following parts: a cylindrical shaft part  96  fitted to the outer circumference of the bearings  92   a  and  92   b ; an idler driven gear  97  that is provided on the outer circumference of the shaft part  96  and meshes with the idler drive gear  61 ; a first-bank-side drive sprocket  98   f  that is provided on the shaft part  96  and meshes with the first-bank-side cam chain  64   f ; and a second-bank-side drive sprocket  98   r  that is provided on the shaft part  96  and meshes with the second-bank-side cam chain  64   r.    
     The first-bank-side cam chain  64   f  is passed around the first-bank-side drive sprocket  98   f  and the intake-side driven sprocket  52  and the exhaust-side driven sprocket  53  in the first bank  12   f . The second-bank-side cam chain  64   r  is passed around the second-bank-side drive sprocket  98   r  and the intake-side driven sprocket  52  and the exhaust-side driven sprocket  53  in the second bank  12   r . That is, the valve train mechanisms  45   f  and  45   r  are driven by the first-bank-side cam chain  64   f  and the second-bank-side cam chain  64   r  driven by one idle gear  62  provided over the crankshaft  10 . By driving the valve train mechanisms  45   f  and  45   r  by the idle gear  62  over the crankshaft  10  in this manner, the first-bank-side cam chain  64   f  and the second-bank-side cam chain  64   r  can be shortened and weight reduction can be achieved. 
     The idler driven gear  97  is provided on the other end side of the idle gear  62  and the second-bank-side drive sprocket  98   r  is provided on the one end side of the idle gear  62 . The first-bank-side drive sprocket  98   f  is provided between the idler driven gear  97  and the second-bank-side drive sprocket  98   r . The first-bank-side drive sprocket  98   f  and the second-bank-side drive sprocket  98   r  have a smaller diameter than the idler driven gear  97 . 
     For the idler driven gear  97 , a sub-gear  99  is so provided as to abut against the outer surface of the idler driven gear  97 . The sub-gear  99  has the same number of teeth and substantially the same diameter as the idler driven gear  97  and is fitted to the shaft part  96 . Between the sub-gear  99  and the idler driven gear  97 , springs  100  extending in the circumferential direction of the idler driven gear  97  are interposed at plural places. Specifically, the spring  100  is provided in both a recess  97   a  made in the idler driven gear  97  and a hole  99   a  made in the sub-gear  99 . Through deflection of the springs  100 , the sub-gear  99  rotates relative to the idler driven gear  97 . 
     A ring-shaped washer  101  fitted to the shaft part  96  is interposed between the sub-gear  99  and the cover-side thrust bearing  94 . The washer  101  is pressed by the cover-side thrust bearing  94  and makes the sub-gear  99  abut against the idler driven gear  97 . Furthermore, the washer  101  is located outside the spring  100  to prevent the removal of the spring  100 . 
     A bearing fitting part  96   a  to which the bearings  92   a  and  92   b  are fitted is provided on the inner circumferential surface of the shaft part  96  and a projection  96   b  that restricts the position of the bearings  92   a  and  92   b  in the axial direction is provided on the inner circumferential surface of one end of the shaft part  96 . When the position of the bearings  92   a  and  92   b  is settled by the projection  96   b , the position of the end surface of the outside bearing  92   a  substantially corresponds with the position of the end surface of the shaft part  96  on the opposite side to the projection  96   b.    
       FIG. 9  is a perspective view showing the part around the crankcase-side boss part  90 . 
     As shown in  FIGS. 6 and 9 , the crankcase-side boss part  90  is provided over the idler drive gear  61  at the lower part of the cam chain chambers  43   f  and  43   r . The crankcase-side boss part  90  is formed into a cylindrical shape protruding from the sidewall  20   b  toward the clutch cover  39  in parallel to the crankshaft  10 . 
     In the sidewall  20   b , a case-side oil passage  58  (oil passage formed in the wall of the crankcase) that is branched from the main gallery  73  and extends in the upward-downward direction in the sidewall  20   b  is made. The inner circumferential part of the crankcase-side boss part  90  serves as an oil passage  105  communicating with the case-side oil passage  58 . 
     The crankcase-side boss part  90  has, at the tip, a substantially flat abutting surface  106  that abuts against the case-side thrust bearing  93  and tip notches  107  to allow the inside of the crankcase-side boss part  90  to communicate with the outside are made in the first abutting surface  106 . The tip notches  107  are made at the upper part and lower part of the crankcase-side boss part  90  as a pair of notches with a positional relationship in which they are opposed to each other. Part of oil pressure-fed to the oil passage  105  passes through the gap between the crankcase-side boss part  90  and the idle shaft  63  and is supplied from the tip notches  107  to the case-side thrust bearing  93 . 
     In the inner circumferential surface of the tip part of the crankcase-side boss part  90 , a shaft fitting part  108  formed with a larger diameter than the back side of the oil passage  105  is made. One end  63   b  of the idle shaft  63  is fitted to the shaft fitting part  108 . The shaft fitting part  108  has, at the bottom part, a step part  108   a  against which the one end  63   b  of the idle shaft  63  is made to abut. 
       FIG. 10  is a perspective view showing a state in which the idle shaft  63  is attached to the crankcase-side boss part  90 . 
     As shown in  FIGS. 6 and 10 , the idle shaft  63  includes a shaft main body part  110  that extends in parallel to the crankshaft  10  and has a circular sectional shape and a shaft-side joint part  111  (joint part) protruding in the axial direction from the shaft main body part  110 . 
     The shaft main body part  110  has an in-shaft oil passage  112  (oil passage) extending along the axial direction at a position substantially corresponding with the position of the axial line of the shaft main body part  110 . The in-shaft oil passage  112  communicates with the oil passage  105  at the one end  63   b . Furthermore, the in-shaft oil passage  112  extends to the vicinity of the shaft-side joint part  111  and ends there. Thus, the in-shaft oil passage  112  does not penetrate the idle shaft  63  in the axial direction at the other end  63   c  of the idle shaft  63 . The inner diameter of the in-shaft oil passage  112  is set smaller than that of the oil passage  105 . Therefore, the idle shaft  63  is formed with a thick wall and its strength and rigidity are ensured. 
     The shaft main body part  110  has plural oil passages  113   a ,  113   b , and  113   c  extending in the radial direction to allow the oil passage  105  to communicate with the outer circumference  63   a . In the assembled state, the oil passage  113   a  is made on the tip side of the other end  63   c . Furthermore, the oil passage  113   b  is made near the idler driven gear  97  and the oil passage  113   c  is made near the first-bank-side drive sprocket  98   f . The shaft main body part  110  is formed longer than the idle gear  62  in the axial direction. 
     A substantially flat end surface  114  is made at the tip part of the shaft main body part  110  on the side of the other end  63   c  and the shaft-side joint part  111  protrudes in the axial direction from the center of the end surface  114 . The shaft-side joint part  111  is formed into a substantially oblong rectangular shape as viewed in the axial direction. 
       FIG. 11  is a plan view of the case-cover-side boss part  91 . 
     As shown in  FIGS. 5, 6, and 11 , the case-cover-side boss part  91  is provided on the inner surface of the side cover part  81  of the clutch cover  39  and is located above the oil chamber  78 . The case-cover-side boss part  91  is formed into a cylindrical shape protruding from the side cover part  81  toward the sidewall  20   b  in parallel to the crankshaft  10 . 
     The case-cover-side boss part  91  has a cylindrical part  120  having a larger diameter than the idle shaft  63  and a fitting hole  121  made at the center of the cylindrical part  120 . 
     As shown in  FIGS. 4 and 5 , the oil passage  85  in the pipe-shaped part  84  is so made as to overlap with the case-cover-side boss part  91  but does not communicate with the fitting hole  121 . 
     The tip part of the cylindrical part  120  has an outside abutting surface  122  (second abutting surface) that protrudes in the axial direction on the outer circumferential side of the cylindrical part  120  and has a circular ring shape in plan view and an inside abutting surface  123  (abutting surface) that protrudes in the axial direction on the inner circumferential side of the cylindrical part  120  and has a circular ring shape in plan view. The outside abutting surface  122  protrudes in the axial direction to a larger extent than the inside abutting surface  123 . 
     Between the outside abutting surface  122  and the inside abutting surface  123 , an oil reservoir  124  that hollows in the axial direction relative to the inside abutting surface  123  and has a circular ring shape in plan view is formed. 
     The outside abutting surface  122  has such a diameter as to overlap with the cover-side thrust bearing  94  as viewed in the axial direction and abuts against the cover-side thrust bearing  94  in the axial direction. The inside abutting surface  123  has such a diameter as to overlap with the bearing  92   a  and abuts against the end surface of the outside bearing  92   a  in the axial direction. The oil reservoir  124  has such a diameter as to overlap with the bearing  92   a , the shaft part  96 , and the cover-side thrust bearing  94  and is opposed to the bearing  92   a , the shaft part  96 , and the cover-side thrust bearing  94  in the axial direction. 
     The inside abutting surface  123  has inside notches  125  (notches) to allow the inside abutting surface  123  to communicate with the oil reservoir  124 . The oil reservoir  124  communicates with the oil passage  113   a  via the inside notches  125 . The inside notches  125  are made at the upper part and lower part of the inside abutting surface  123  as a pair of notches with a positional relationship in which they are substantially opposed to each other as viewed in the axial direction. 
     The outside abutting surface  122  has outside notches  126  (second notches) to allow the oil reservoir  124  to communicate with a space outside the case-cover-side boss part  91 . The outside notches  126  are made at the left and right side parts of the outside abutting surface  122  as a pair of notches with a positional relationship in which they are substantially opposed to each other as viewed in the axial direction. A straight line L 1  linking the pair of inside notches  125  is substantially orthogonal to a straight line L 2  linking the pair of outside notches  126 . 
     The part of the shaft main body part  110  of the idle shaft  63  on the side of the end surface  114  is fitted to the fitting hole  121  of the case-cover-side boss part  91  and the shaft-side joint part  111  is located in the fitting hole  121 . 
     The fitting hole  121  has a cover-side joint part  128  (joint part) with which the shaft-side joint part  111  engages in a bottom part  127  of the hole. The cover-side joint part  128  is a substantially rectangular groove that passes through the center of the bottom part  127  having a substantially circular shape in plan view and extends along the upward-downward direction. Through fitting of the shaft-side joint part  111  to the cover-side joint part  128 , the idle shaft  63  is connected to the case-cover-side boss part  91 . The cover-side joint part  128  overlaps with the straight line L 1  and extends along the upward-downward direction. 
     The spring  95  is so disposed as to be fitted to the outer circumference of the shaft-side joint part  111  in the fitting hole  121  and is compressed between the bottom part  127  and the end surface  114  of the idle shaft  63 . 
     When the idle gear  62  is assembled to the crankcase  11 , first a small assembly is formed by temporarily assembling, to the idle shaft  63 , the bearings  92   a  and  92   b , the idle gear  62 , the sub-gear  99 , the spring  100 , the washer  101 , the cover-side thrust bearing  94 , the case-side thrust bearing  93 , and the spring  95 . 
     Subsequently, the small assembly is temporarily assembled to the crankcase-side boss part  90  in such a manner that the one end  63   b  of the idle shaft  63  is fitted to the shaft fitting part  108  of the crankcase-side boss part  90  and the one end  63   b  is made to abut against the step part  108   a . Therefore, the idle shaft  63  abuts against the step part  108   a  and the position thereof is settled in the axial direction. In addition, the idler driven gear  97  and the sub-gear  99  mesh with the idler drive gear  61 . The end surface  114  of the idle shaft  63  and the shaft-side joint part  111  protrude outward relative to the outer end of the shaft part  96 . Moreover, the first-bank-side cam chain  64   f  is wound around the first-bank-side drive sprocket  98   f  and the second-bank-side cam chain  64   r  is wound around the second-bank-side drive sprocket  98   r.    
     Next, the clutch cover  39  is fixed to the crankcase  11  from the outside by bolts (not shown) inserted into the plural fixing holes  39   a . Specifically, in the assembling of the clutch cover  39 , the other end  63   c  of the idle shaft  63  is fitted to the fitting hole  121  of the case-cover-side boss part  91  and the shaft-side joint part  111  is connected to the cover-side joint part  128 . That is, the idle shaft  63  is supported through the fitting of the one end  63   b  and the other end  63   c  to the crankcase-side boss part  90  and the case-cover-side boss part  91 . In addition, the idle shaft  63  is so fixed as to be incapable of rotating due to the connecting of the shaft-side joint part  111  to the cover-side joint part  128 . Furthermore, the position of the idle shaft  63  is settled in the rotational direction due to the connecting of the shaft-side joint part  111  to the cover-side joint part  128 . This allows the oil passage  113   a  to communicate with the inside notches  125 . 
     Between the tip surface of the shaft-side joint part  111  and the bottom surface of the cover-side joint part  128 , a gap S is set in the axial direction. The spring  95  is compressed between the bottom part  127  and the end surface  114  and biases the idle shaft  63  toward the crankcase-side boss part  90  so that the gap S can be ensured. As shown in  FIG. 6 , the idle shaft  63  receives a force in such a direction as to move toward the clutch cover  39  due to an oil flow F flowing from the oil passage  105  to the in-shaft oil passage  112 . 
     In the present embodiment, the inner diameter of the in-shaft oil passage  112  is set smaller than that of the oil passage  105  to ensure the strength. Therefore, the idle shaft  63  is susceptible to the influence of the oil flow F on the side of the one end  63   b . However, because being biased toward the crankcase-side boss part  90  by the spring  95 , the idle shaft  63  can be prevented from moving to a large extent in the axial direction due to the oil flow F. Thus, even when there is variation in the oil flow F, the vibration of the idle shaft  63  in the axial direction can be prevented, so that generation of hammering noises can be prevented. Moreover, because the gap S is set between the tip surface of the shaft-side joint part  111  and the bottom surface of the cover-side joint part  128 , the dimensional accuracy of the shaft-side joint part  111  and the cover-side joint part  128  in the axial direction does not need to be strictly managed and therefore the manufacturing is easy. 
     Furthermore, through the slight movement of the idle shaft  63  in the axial direction against the spring  95  due to the oil flow F, the gap between the crankcase-side boss part  90  and the idle shaft  63  becomes larger. This increases the amount of oil that passes through the gap and flows to the tip notches  107  and thus can effectively supply the oil to the case-side thrust bearing  93 . 
     In the state in which the clutch cover  39  is attached, the outside abutting surface  122  of the case-cover-side boss part  91  abuts against the cover-side thrust bearing  94  and the inside abutting surface  123  abuts against the end surface of the bearing  92   a . That is, the outside abutting surface  122  and the inside abutting surface  123  press the idle shaft  63  toward the crankcase-side boss part  90  through the cover-side thrust bearing  94  and the bearing  92   a . By this pressing force, the case-side thrust bearing  93  is clamped between the first abutting surface  106  and the idle gear  62 . 
     The idle gear  62  is supported by the case-side thrust bearing  93  and the cover-side thrust bearing  94  in the axial direction and is supported by the bearings  92   a  and  92   b  in the radial direction, and rotates around the idle shaft  63 . 
     The washer  101  is pressed against the sub-gear  99  by the cover-side thrust bearing  94 . 
     The oil supplied from the oil passage  105  to the side of the idle shaft  63  flows as shown by arrows in  FIG. 6 . Specifically, part of the oil supplied from the oil passage  105  to the in-shaft oil passage  112  passes through the oil passages  113   b  and  113   c  and is supplied to the bearings  92   a  and  92   b . Furthermore, part of the oil in the in-shaft oil passage  112  passes from the oil passage  113   a  and through the inside notches  125  to flow into the oil reservoir  124 . The oil in the oil reservoir  124  is supplied to the bearings  92   a  and  92   b , the cover-side thrust bearing  94 , and the idler driven gear  97 . In addition, the oil passes through the outside notches  126  and is discharged to the space outside the case-cover-side boss part  91  to drop down and return to the oil pan  40 . Specifically, the oil in the oil reservoir  124  flows into the bearing  92   a  from a gap on the side of the end surface of the bearing  92   a  in the axial direction. 
     As shown in  FIG. 11 , in the case-cover-side boss part  91 , the outside notches  126  are made at the left and right side parts of the outside abutting surface  122 . Thus, a large amount of oil can be accumulated in the oil reservoir  124 . Furthermore, the inside notches  125  of the inside abutting surface  123  are made at different positions in the circumferential direction from the outside notches  126  in such a manner that the straight line L 1  is substantially orthogonal to the straight line L 2 . Therefore, oil supplied from the inside notches  125  to the oil reservoir  124  can be prevented from being immediately discharged from the outside notches  126  and the oil can be retained in the oil reservoir  124  evenly. Thus, the oil can be properly fed to oil feed points. 
     As described above, according to the embodiment to which the present invention is applied, the valve train system drive device  60  is provided in the engine  1  having the crankshaft  10  provided in the crankcase  11  and the valve train mechanisms  45   f  and  45   r , and has the idle gear  62  that transmits power received from the crankshaft  10  to the valve train mechanisms  45   f  and  45   r , the idle shaft  63  that is provided in parallel to the crankshaft  10  and rotatably supports the idle gear  62 , and the clutch cover  39  that covers the crankcase  11  from a lateral side. The crankcase-side boss part  90  that is formed on the sidewall  20   b  of the crankcase  11  and supports the one end  63   b  of the idle shaft  63  and the case-cover-side boss part  91  that is formed inside the clutch cover  39  and supports the other end  63   c  of the idle shaft  63  are provided. The cover-side joint part  128  and the shaft-side joint part  111  that engage with each other are formed at the case-cover-side boss part  91  and the other end  63   c  of the idle shaft  63 , and engagement of the cover-side joint part  128  and the shaft-side joint part  111  with each other precludes the idle shaft  63  from rotating. By this configuration, the one end  63   b  and the other end  63   c  of the idle shaft  63  are supported by the crankcase-side boss part  90  formed on the sidewall  20   b  of the crankcase  11  and the case-cover-side boss part  91  formed on the clutch cover  39 . In addition, the idle shaft  63  can be made incapable of rotating by the shaft-side joint part  111  and the cover-side joint part  128  on the side of the other end  63   c . Thus, the structure to support the idle shaft  63  can be simplified. This allows size reduction, weight reduction, and simplification of the engine  1 . 
     The idle gear  62  is rotatably supported by the idle shaft  63  with the intermediary of the bearings  92   a  and  92   b , and the in-shaft oil passage  112  is formed in the idle shaft  63 , and the in-shaft oil passage  112  is connected to the case-side oil passage  58  formed in the sidewall  20   b  of the crankcase  11  at the one end  63   b  of the idle shaft  63 . Furthermore, the case-cover-side boss part  91  has the inside abutting surface  123  that abuts against the bearings  92   a  and  92   b  in the axial direction and the oil reservoir  124  having a concave shape is provided at the outer periphery of the inside abutting surface  123 . Thus, oil can be supplied to the bearings  92   a  and  92   b  from the oil reservoir  124  on the outer peripheral side of the inside abutting surface  123  of the case-cover-side boss part  91  and therefore the bearings  92   a  and  92   b  can be fed with oil by a simple structure. 
     The inside notches  125  to allow the inside abutting surface  123  to communicate with the oil reservoir  124  are made in the inside abutting surface  123  and the oil passage  113   a  in the idle shaft  63  communicates with the oil reservoir  124  due to the inside notches  125 . Therefore, the oil in the oil reservoir  124  can be sufficiently supplied to the bearings  92   a ,  92   a  via the inside notches  125 . 
     The outside abutting surface  122  is provided at the outer periphery of the oil reservoir  124  and the outside abutting surface  122  protrudes in the axial direction relative to the inside abutting surface  123 . Therefore, the capacity of the oil reservoir  124  can be set large, which allows effective oil feed to the bearings  92   a  and  92   b.    
     The cover-side thrust bearing  94  is interposed between the outside abutting surface  122  and the idle gear  62 . Thus, the cover-side thrust bearing  94  can be fed with oil from the oil reservoir  124 . 
     The outside notches  126  to allow the oil reservoir  124  to communicate with a space outside the case-cover-side boss part  91  are made in the outside abutting surface  122 . Therefore, when excess oil is supplied to the oil reservoir  124 , the oil can be discharged from the outside notches  126  to the space outside the case-cover-side boss part  91 . 
     The inside notches  125  and the outside notches  126  are made at positions different from each other on the case-cover-side boss part  91 . Thus, oil can be evenly supplied to the oil reservoir  124  and the capacity of the oil reservoir  124  can be efficiently used. 
     The inside notches  125  are made as a pair of notches with placement in which the inside notches  125  are opposed to each other in the radial direction on the annular inside abutting surface  123 , and the outside notches  126  are made as a pair of notches with placement in which the outside notches  126  are opposed to each other on the annular outside abutting surface  122 . Furthermore, the straight line L 1  linking the pair of inside notches  125  is substantially orthogonal to the straight line L 2  linking the pair of outside notches  126 . Thus, oil can be evenly supplied to the oil reservoir  124  and the capacity of the oil reservoir  124  can be efficiently used. 
     The in-shaft oil passage  112  is formed in the idle shaft  63 , and the in-shaft oil passage  112  is connected to the case-side oil passage  58  formed in the sidewall  20   b  of the crankcase  11  at the one end  63   b  of the idle shaft  63 , and the spring  95  is interposed between the other end  63   c  of the idle shaft  63  and the case-cover-side boss part  91 . Therefore, even when variation is caused in the oil pressure of oil passing through the in-shaft oil passage  112  in the idle shaft  63 , the idle shaft  63  can be supported in the axial direction by the spring  95  and generation of sounds attributed to the vibration of the idle shaft  63  can be prevented. 
     In the above embodiment, it is explained that the straight line L 1  linking the pair of inside notches  125  is substantially orthogonal to the straight line L 2  linking the pair of outside notches  126 . However, the invention of the present application is not limited thereto and the positions of the notches may be changed. A description will be made below about this case as a modification example. In this modification example, parts formed in the same manner as the above embodiment are given the same symbols and description thereof is omitted. 
     MODIFICATION EXAMPLE 
       FIG. 12  is a plan view of a case-cover-side boss part  291  in the modification example according to a second embodiment of the invention. 
     The case-cover-side boss part  291  has the cylindrical part  120 , the fitting hole  121 , the outside abutting surface  122 , the inside abutting surface  123 , the oil reservoir  124 , the bottom part  127 , and the cover-side joint part  128 . 
     The inside abutting surface  123  has an inside notch  225  to allow the inside abutting surface  123  to communicate with the oil reservoir  124 . The oil reservoir  124  communicates with the oil passage  113   a  via the inside notch  225 . The inside notch  225  is at a side part of the inside abutting surface  123  as viewed in the axial direction. 
     The outside abutting surface  122  has an outside notch  226  (second notch) to allow the oil reservoir  124  to communicate with a space outside the case-cover-side boss part  291 . The outside notch  226  is made at a side part of the outside abutting surface  122  at a position different from that of the inside notch  225  by substantially  180  degrees in the circumferential direction as viewed in the axial direction. 
     In the case-cover-side boss part  291 , the outside notch  226  is made at a side part of the outside abutting surface  122  in the left-right direction and thus a large number of oil can be accumulated in the oil reservoir  124 . Furthermore, the inside notch  225  of the inside abutting surface  123  is made at a position that is different from that of the outside notch  226  by substantially  180  degrees in the circumferential direction and is distant from the outside notch  226 . Therefore, oil supplied from the inside notch  225  to the oil reservoir  124  can be prevented from being immediately discharged from the outside notch  226 . Thus, the oil can be retained in the oil reservoir  124  evenly and the oil can be properly fed to oil feed points. 
     The above embodiment is what shows one aspect to which the present invention is applied and the present invention is not limited to the above embodiment. 
     In the above embodiment, the description is made by taking as an example the spring  95  having a coil shape as the elastic spring member that biases the idle shaft  63  in the axial direction. However, the configuration is not limited thereto and the idle shaft  63  may be biased in the axial direction by an elastic spring member such as rubber for example. 
     Furthermore, in the above embodiment, it is explained that a small assembly is formed in assembling the idle gear  62  to the crankcase  11 . However, the configuration is not limited thereto and the respective parts may be individually assembled. 
     Although the present invention has been described herein with respect to a number of specific illustrative embodiments, the foregoing description is intended to illustrate, rather than to limit the invention. Those skilled in the art will realize that many modifications of the illustrative embodiment could be made which would be operable. All such modifications, which are within the scope of the claims, are intended to be within the scope and spirit of the present invention. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
           1  Engine 
           10  Crankshaft 
           11  Crankcase 
           20   b  Sidewall (wall of crankcase) 
           39  Clutch cover (case cover) 
           45   f ,  45   r  Valve train mechanism (valve train system) 
           58  Case-side oil passage (oil passage formed in wall of crankcase) 
           60  Valve train system drive device 
           62  Idle gear 
           63  Idle shaft 
           63   b  One end 
           63   c  The other end 
           90  Crankcase-side boss part 
           91 ,  291  Case-cover-side boss part 
           92   a ,  92   b  Bearing (bearing member) 
           94  Cover-side thrust bearing (thrust bearing) 
           95  Spring (elastic spring member) 
           111  Shaft-side joint part (joint part) 
           112  In-shaft oil passage (oil passage) 
           122  Outside abutting surface (second abutting surface) 
           123  Inside abutting surface (abutting surface) 
           124  Oil reservoir 
           125 ,  225  Inside notch (notch) 
           126 ,  226  Outside notch (second notch) 
           128  Cover-side joint part (joint part) 
         L 1  Straight line (straight line linking a pair of notches) 
         L 2  Straight line (straight line linking a pair of second notches)