Abstract:
To deem a dedicated oil passage unnecessary and therefore make an engine equipped with a plurality of cylinder bores with substantially horizontal axes and provided with transmission mechanisms between a camshaft and crankshaft within a valve chamber more lightweight and compact while returning oil for a valve chamber to a crank chamber. Transmission mechanisms include endless chains wrapped around drive sprockets fixed to one end of a crankshaft and driven sprockets fixed to one end of camshafts. The transmission mechanisms are housed in transmission chambers in such a manner that ends of the transmission mechanisms are made to communicate with the valve chambers and the other ends of the transmission mechanisms are made to face one end in the axial direction of the crankshaft. A return hole causing the lower part of the other end of the transmission chambers to communicate with the crank chamber is provided at the crankcases.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oil passage structure for returning oil in an engine. A plurality of cylinder bores with substantially horizontal axes are lined up at cylinder blocks. A plurality of combustion chambers individually corresponding to each cylinder bore are formed between pistons fitted in a freely slidable manner into each cylinder bore and cylinder heads. Cam shafts interlocking with and coupled to a plurality of intake valves and a plurality of exhaust valves controlling intake of air to and exhaust emissions from the combustion chambers are positioned within valve chambers formed between the cylinder heads and head covers coupled to the cylinder heads. Furthermore, transmission mechanisms for reducing the rotational force of the crankshaft by half for transmission to the camshafts are provided between the crankshaft to which the pistons are commonly coupled at the crank chamber within the crankcases coupled to the cylinder blocks and the camshafts. More particularly, the present invention relates to an oil passage structure for returning oil from a valve chamber to a crank chamber. 
     2. Description of Related Art 
     Conventionally, as disclosed in, for example, Utility Model Laid-open Publication No. Sho. 63-65811, an oil passage for returning oil is typically provided in a cylinder head and cylinder block of an engine. 
     In the related art, a dedicated oil passage for returning oil is provided in the cylinder block and cylinder head. The cylinder block and cylinder head therefore become larger as a result of space being provided to position this oil passage for returning oil. Furthermore, the weight of the engine is increased. 
     SUMMARY OF THE INVENTION 
     In order to resolve the aforementioned situation, it is the object of the present invention to provide an oil passage structure for returning oil in an engine which is both lighter and more compact due to a dedicated oil passage no longer being required. Furthermore, it is an object of the present invention for oil in a valve chamber to be returned to a crank chamber. 
     In order to achieve the aforementioned object, according to a first aspect of the present invention, an oil passage structure for returning oil in an engine includes a plurality of cylinder bores with substantially horizontal axes lined up at cylinder blocks, a plurality of combustion chambers individually corresponding to each cylinder bore formed between pistons fitted in a freely slidable manner into each cylinder bore and cylinder head, cam shafts interlocking with and coupled to a plurality of intake valves and a plurality of exhaust valves controlling intake of air to and exhaust emissions from the combustion chambers positioned within valve chambers formed between the cylinder heads and head covers coupled to the cylinder heads, and transmission mechanisms for reducing the rotational force of the crankshaft by half for transmission to the camshafts provided between the crankshaft to which the pistons are commonly coupled at the crank chamber within the crankcases coupled to the cylinder blocks and the camshafts. The transmission mechanisms include endless chains wrapped around drive sprockets fixed to one end of the crankshaft and driven sprockets fixed to one end of the camshafts being housed in transmission chambers formed across the head covers, cylinder heads, cylinder blocks and crankcases in such a manner that ends of the transmission mechanisms are made to communicate with the valve chambers and the other ends of the transmission mechanisms are made to face one end in the axial direction of the crankshaft. Furthermore, a return hole causing the lower part of the other end of the transmission chambers to communicate with the crank chamber is provided at the crankcases. 
     According to this configuration, it is possible to make oil circulate about the transmission chamber housing the transmission mechanism because the transmission mechanism uses a chain to transmit force. Oil can therefore be returned to the crank chamber from the valve chamber via the return hole using the transmission chamber housing the transmission mechanisms provided between the camshaft and the crankshaft. As it is therefore no longer necessary to provide a dedicated oil returning oil passage at the cylinder blocks and the cylinder heads for at least the flow of oil returning to the crank chamber from the transmission chamber, the cylinder blocks and cylinder heads can be made smaller accordingly and can also be made more lightweight. 
     According to a second aspect of the present invention, in addition to the configuration of the first aspect of the present invention, the crankshaft is supported in a freely rotatable manner by the crankcase and bearing caps fastened to the crankcase at a plurality of places along the axial direction of the crankshaft. The return hole is formed so as to be extended in the direction of fastening the bearing caps to the crankcase. According to this configuration, the surface area of the opening of the return hole can be set to be relatively large without damaging the rigidity with which the crankshaft is supported and the extent to which oil is returned to the crank chamber is improved. 
     According to a third aspect of the present invention, in addition to the configuration of the second aspect of the present invention, the pair of cylinder blocks equipped with pluralities of cylinder bores positioned opposite each other with substantially horizontal axes on both sides of the crankshaft are established at the pair of mutually fastened crankcases defining the crank chamber. The bearing caps are fastened to a journal wall integrally formed with one of the crankcases. Furthermore, the return hole is provided between the crankcases positioned offset towards the side of one of the crankcases. 
     According to this configuration, in a horizontally opposed engine where a pair of cylinder bore rows face each other in a substantially horizontal plane, lowering of the rigidity of a crankcase on a side where a journal wall supporting the crankshaft together with bearing caps is not integrally formed is avoided, the surface area of the return hole can be made larger and the extent to which oil is returned to the crank chamber is dramatically improved. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a side view of a horizontally-opposed type engine mounted on a motorcycle; 
     FIG. 2 is a front view in the direction of the arrow  2  of FIG. 1; 
     FIG. 3 is an enlarged sectional view taken on line  3 — 3  of FIG. 1; 
     FIG. 4 is a sectional view taken on line  4 — 4  of FIG. 3; 
     FIG. 5 is an enlarged view taken on line  5 — 5  of FIG. 4; 
     FIG. 6 is an enlarged view taken on line  6 — 6  of FIG. 4; 
     FIG. 7 is an enlarged sectional view taken on line  7 — 7  of FIG. 4; 
     FIG. 8 is an enlarged view taken on line  8 — 8  of FIG. 4; 
     FIG. 9 is an enlarged sectional view taken on line  9 — 9  of FIG. 4; 
     FIG. 10 is a sectional view taken on line  10 — 10  of FIG. 8; 
     FIG. 11 is a sectional view taken on line  11 — 11  of FIG. 3; 
     FIG. 12 is a sectional view taken on line  12 — 12  of FIG. 11; 
     FIG. 13 is a schematic view from the rear side of a transmission case; and 
     FIG. 14 is an enlarged sectional view taken on line  14 — 14  of FIG.  13 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, one embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIGS. 1 to  14  show one embodiment of the present invention. Referring first to FIGS. 1 and 2, a four-cycle/multi-cylinder (e.g., six-cylinder) horizontally-opposed type engine is mounted on a motorcycle. An engine main body E of the engine includes a left engine block B L  disposed on the left side when the motorcycle is directed forwardly in the running direction thereof, and a right engine block B R  disposed on the right side in when the motorcycle is directed forwardly in the running direction thereof. 
     Referring particularly to FIGS. 3 and 4, the left engine block B L  includes a left cylinder block  23   L  and a left cylinder head  24   L  connected to the left cylinder block  23   L . The left cylinder block  23   L  has a left side cylinder bore row  22   L including a plurality (e.g., three) of cylinder bores  21   L  disposed in parallel. The left cylinder head  24   L  has combustion chambers  26   L  each of which is formed between the associated one of the cylinder bores  21   L  and a piston  25   L  slidably fitted in the cylinder bore  21   L . A left crank case  27   L  is formed integrally with the side, opposed to the left cylinder head  24   L , of the cylinder block  23   L . The right engine block B R  includes a right cylinder block  23   R  and a right cylinder head  24   R  connected to the right cylinder block  23   R . The right cylinder block  23   R  has a right side cylinder bore row  22   R  including a plurality (e.g., three) of cylinder bores  21   R  disposed in parallel. The right cylinder head  24   R  has combustion chambers  26   R  each of which is formed between the associated one of the cylinder bores  21   R  and a piston  25   R  slidably fitted in the cylinder bore  21   R . A right crank case  27   R  is formed integrally with the side, opposed to the right cylinder head  24   R , of the cylinder block  23   R . 
     The left and right engine blocks B L  and B R  are opposed to each other with the axial lines of the cylinder bores  21   L  and  21   R  directed substantially in the horizontal direction. The left crank case  27   L  of the left engine block B L  is fastened to the right crank case  27   R  of the right engine block B R  in such a manner as to form a crank chamber  28  therebetween. 
     The pistons  25   L  and  25   R  in the left and right engine blocks B L  and B R  are commonly connected to a crank shaft  29  via connecting rods  30   L  and  30   R , respectively. The crank shaft  29  is disposed such that one end side is located on the front side of the motorcycle in the longitudinal direction of the motorcycle and the axial line of the crank shaft  29  extends in the longitudinal direction of the motorcycle. The crank shaft  29  is supported by one of the left and right crank cases  27   L  and  27   R  (left crank case  27   L  in this embodiment). To be more specific, the crank shaft  29  is rotatably supported by journal walls  31  integrally formed on the left crank case  27   L  at a plurality of locations spaced in the axial direction of the crank shaft  29 . Furthermore, bearing caps  32  are fastened to the journal walls  31  with a pair of bolts  33 , respectively. 
     Each of the cylinder bores  21   R  constituting the cylinder bore row  22   R  on the right engine block B R  side is offset forwardly in the longitudinal direction of the motorcycle from the associated one of the opposed cylinder bores  21   L  constituting the cylinder bore row  22   L  on the left engine block B L  side by a first offset amount L1. 
     Referring particularly to FIGS. 5,  6  and  7 , the left cylinder head  24   L  includes pairs of intake passages  34   L  and exhaust passages  35   L  communicating with the combustion chambers  26   L . Each pair of the intake passages  34   L  and the exhaust passages  35   L  are provided for the associated one of the combustion chambers  26   L . The left cylinder head  24   L  also includes intake valves  36   L  each being adapted to open/close the associated one of the intake passages  34   L  and exhaust valves  37   L  each being adapted to open/close the associated one of the exhaust passages  35   L  The intake valves  36   L  and the exhaust valves  37   L , which extend in the direction parallel to the axial line of the crank shaft  29 , are offset upwardly from a plane  38   L  passing through the axial lines of the cylinder bores  21   L  and the axial line of the crank shaft  29  in such a manner that the exhaust valves  37   L  are offset forwardly from the intake valves  36   L  in the longitudinal direction of the motorcycle. The left cylinder head  24   L  also includes ignition plugs  39   L  facing toward the central portion of an associated one of the combustion chambers  26   L  at a position located between an associated one of the pairs of the intake valves  36   L  and exhaust valve  37   L  on an opposite side from the disposition side of the intake valves  36   L  and the exhaust valves  37   L  with respect to the plane  38   L . In other words, the ignition plugs are located on the lower side of the plane  38   L . 
     Each of the intake valves  36   L  and the exhaust valves  37   L  is mounted to the left cylinder head  24   L  in such a manner as to be tilted at an acute angle with respect to the plane  38   L . On the opposite side from the disposition side of the intake valves  36   L  and the exhaust valves  37   L  with respect to the plane  38   L , i.e., on the lower side of the plane  38   L , the left cylinder head  24   L  has plug mounting holes  40   L  for mounting the ignition plugs  39   L  in a state where the ignition plugs  39   L  are tiled at an acute angle with respect to the plane  38   L . In other words, the ignition plugs  39   L  are mounted to the left cylinder head  24   L  in such a manner as to be tilted downwardly with respect to the plane  38   L . 
     On the projection chart crossing the axial lines of the cylinder bores  21   L  at right angles, the intake passages  34   L  are provided in the left cylinder head  24   L  in such a manner as to cross the plane  38   L  substantially at right angles, and are opened to one side surface of the left cylinder head  24   L  on the disposition side of the intake valves  36   L  and the exhaust valves  37   L  with respect to the plane  38   L , i.e., on the upper side of the plane  38   L . The exhaust passages  35   L  are opened to the other side surface of the left cylinder head  24   L  on an opposite side from the disposition side of the intake valves  36   L  and the exhaust valves  37   L  with respect to the plane  38   L , i.e., on the lower side of the plane  38   L . To be more specific, the exhaust passages  35   L are curved to be swelled toward one end side of the crank shaft  29  or the front side of the motorcycle in order to bypass the ignition plugs  39   L , that is, the plug mounting holes  40   L  for mounting the ignition plugs  39   L . 
     Each of the exhaust passages  35   L  is formed in such a manner as to be tilted downwardly toward the central portion of the motorcycle in the width direction and to be opened to the other side surface, i.e., the lower surface of the left cylinder head  24   L . An exhaust system  43   L  is provided which is composed exhaust pipes  41   L  each of which is in communication with an associated one of the exhaust passages  35   L , a catalyst converter  42 , an exhaust muffler (not shown), and the like. Each of the exhaust pipes  41   L  of the exhaust system  43   L  is tilted such that it is closer to the central portion of the motorcycle in the width direction since it is separated apart downwardly from the left cylinder head  24   L , and is connected to an opening at the outer end of the associated one of the exhaust passages  35   L . 
     The center of the opening at the outer end of each exhaust passage  35   L  is offset forwardly in the longitudinal direction of the motorcycle from a center C L  of an associated one of the combustion chambers  26   L  by a second offset amount L2. 
     A single cam shaft  46   L , which is in parallel to the crank shaft  29  and has an axial line perpendicular to the opening/closing operational lines of the intake valves  36   L  and the exhaust valves  37   L , is disposed on the disposition side of the intake valves  36   L  and the exhaust valves  37   L  with respect to the plane  38   L . In other words, the single cam shaft  46   L  is on the upper side of the plane  38   L . On the other hand, the upper ends of the intake valves  36   L  and the exhaust valves  37   L  biased in the valve closing direction, i.e., upwardly, by springs are in contact with valve lifters  47   L  which are supported by the left cylinder head  24   L  slidably in the direction of the operational axial lines of the valves  36   L  and  37   L . The cam shaft  46   L  includes intake side cams  48   L  in contact with the valve lifters  47   L  associated with the intake valves  36   L . Exhaust side cams  49   L  are in contact with the valve lifters  47   L  associated with the exhaust valves  37   L . In other words, the intake valves  36   L  and the exhaust valves  37   L  are directly opened/closed by the intake side cams  48   L  and the exhaust side cams  49   L  of the cam shaft  46   L , respectively. 
     A plurality (for example, four) of portions, spaced in the axial line direction, of the cam shaft  46   L  are rotatably supported by cam bearing portions  50   L  provided on the left cylinder head  24   L  and a cam holder  51   L  commonly fastened to the cam bearing portions  50   L . Of the four cam bearing portions  50   L , three are each provided on the left cylinder head  24   L  in such a manner as to be disposed between a pair of the intake valves  36   L  and the exhaust valves  37   L  provided for each combustion chamber  26   L . The remaining cam bearing portion  50   L  is provided on the left cylinder head  24   L  in such a manner as to be located outside of the combustion chamber  26   L  disposed at the outermost end on one end side of the cam shaft  46   L  (front end side of the motorcycle). 
     An oil passage  52   L  having both ends closed is coaxially provided in the cam shaft  46   L . As shown in FIG. 3, the cam shaft  46   L  has oiling holes  53   L  at positions corresponding to the cam bearing portions  50   L . The oiling holes  53   L  are formed in such a manner as to extend from the inside to the outside of the cam shaft  46   L . Accordingly, lubricating oil is supplied from the interior of the cam shaft  46   L  to the cam bearing portions  50   L  and the cam holder  51   L . Furthermore, an oil groove  54   L  facing to the outer surface of the cam shaft  46   L  is provided in the cam bearing portion  50   L  disposed at the outermost end on one end side of the cam shaft  46   L , and an oiling passage  55   L  provided in the left cylinder head  24   L  and the left cylinder block  23   L  is in communication with the oil groove  54   L . Accordingly, oil is supplied from the oiling passage  55   L  into the oil passage  52   L  in the cam shaft  46   L  via the oil groove  54   L  and the oiling hole  53   L . 
     Each of the intake side cams  48   L  and the exhaust side cams  49   L  has an oiling hole (not shown) communicating with the oil passage  52   L  in the cam shaft  46   L . The outer end of the oiling hole is opened to the outer surface of an associated one of the intake side cams  48   L  and the exhaust side cams  49   L . Accordingly, lubricating oil is also supplied to a slide-contact portion between each of the intake side cams  48   L  and the exhaust side cams  49   L  and the valve lifters  47   L  provided for each of the intake valves  36   L  and the exhaust valves  37   L . 
     The left cylinder head  24   L  is fastened at a plurality of locations to the left cylinder block  23   L . On the opposite side from the disposition side of the intake valves  36   L  and the exhaust valves  37   L  with respect to the plane  38   L , i.e., on the lower side of the plane  38   L , the left cylinder head  24   L  has a plurality (for example, four) of through-holes  56   L  spaced in the axial line direction of the cam shaft  46   L . Of the four through-holes  56   L , two are each disposed between adjacent ones of the combustion chambers  26   L . Fastening bolts  57   L  for fastening the left cylinder head  24   L  to the left cylinder block  23   L  are inserted in the through-holes  56   L . 
     Each through-hole  56   L  is adjacent, on one end side (left side in FIG. 7) of the cam shaft  46   L , to an associated one of the exhaust passages  35   L  bypassing the ignition plugs  39   L  provided for the combustion chambers  26   L . The through-hole  56   L  has a positional relationship such that a distance L4 between a center of the through-hole  56   L  and a center C L  of the associated combustion chamber  26   L  is larger than a value L3 (L3&lt;L4). The value L3 is half a distance (2L3) between the centers C L  of adjacent ones of the combustion chambers  26   L . 
     On the disposition side of the intake valves  36   L  and the exhaust valves  37   L  with respect to the plane  38   L , i.e., on the upper side of the plane  38   L , the left cylinder head  24   L  has a plurality (for example, four) of through-holes  58   L  spaced in the axial line direction of the cam shaft  46   L . Of the four through-holes  58   L , two are each disposed between adjacent ones of the combustion chambers  26   L . Fastening bolts  59   L  for fastening the left cylinder head  24   L  to the left cylinder block  23   L  are inserted in the through-holes  58   L . Each through-hole  58   L , i.e., fastening bolt  59   L  is disposed at a position where it is partially covered by the cam shaft  46   L . 
     A left head cover  60   L  is fastened to the left cylinder head  24   L  in such a manner that a valve system chamber  61   L  for containing the cam shaft  46   L  and the cam holder  51   L  is formed between the left head cover  50   L  and the left cylinder head  24   L . Since the cam shaft  46   L  is disposed upwardly from the plan  38   L  containing the axial lines of the cylinder bores  21   L , the valve system chamber  61   L  is also formed between the left head cover SOL and the left cylinder head  24   L  in such a manner as to be offset upwardly from the plane  38   L . 
     A cover portion  62   L  is formed integrally with the left head cover  60   L . Portions of the exhaust pipes  41   L  of the exhaust system  43   L  connected to the exhaust passages  35   L , and the ignition plugs  39   L  disposed downwardly therefrom are covered from the outside by the cover portion  62   L . 
     Referring particularly to FIGS. 8 and 9, the right cylinder head  24   R  includes pairs of intake passages  34   R  and exhaust passages  35   R  communicating with the combustion chambers  26   R , each pair being provided for an associated one of the combustion chambers  26   R . The right cylinder head  24   R  also includes intake valves  36   R  each being adapted to open/close an associated one of the intake passages  34   R  and exhaust valves  37   R  each being adapted to open/close the associated one of the exhaust passages  35   R . 
     The intake valves  36   R  and the exhaust valves  37   R , which extend in the direction parallel to the axial line of the crank shaft  29 , are offset upwardly from a plane  38   R  passing through the axial lines of the cylinder bores  21   R  and the axial line of the crank shaft  29  in such a manner that the exhaust valves  37   R  are offset forwardly from the intake valves  36   R  in the longitudinal direction of the motorcycle. Ignition plugs  39   R , each of which faces to the central portion of an associated one of the combustion chambers  26   R , are mounted to the right cylinder head  24   R  on a lower side of the plane  38   R . 
     Each of the intake valves  36   R  and the exhaust valves  37   R  is tilted at an acute angle with respect to the plane  38   R . On the lower side from the plane  38   R , the right cylinder head  24   R  has plug mounting holes  40   R  for mounting the ignition plugs  39   R  in a state where the ignition plugs  39   R  are tilted at an acute angle with respect to the plane  38   R . The ignition plugs  39   R  are thus mounted to the right cylinder head  24   R  in such a manner as to be tilted downwardly with respect to the plane  38   R . 
     On the projection chart crossing the axial lines of the cylinder bores  21   R  at right angles, the intake passages  34   R  are provided in the right cylinder head  24   R  in such a manner as to cross the plane  38   R  substantially at right angles, and are opened to one side surface of the right cylinder head  24   R  on the upper side of the plane  38   R . The exhaust passages  35   R  are opened to the other side surface of the right cylinder head  24   R  on the lower side from the plane  38   R . To be more specific, the exhaust passages  35   R  are curved to be swelled toward one end side of the crank shaft  29  in the axial direction or the front side of the motorcycle in order to bypass the ignition plugs  39   R , that is, the plug mounting holes  40   R . 
     Each of the exhaust passages  35   R  is formed in such a manner as to be tilted downwardly toward the central portion of the motorcycle in the width direction and to be opened to the lower surface of the right cylinder head  24   R . An exhaust system  43   R  is provided which is composed of exhaust pipes  41   R , each of which is in communication with an associated one of the exhaust passages  35   R , a catalyst converter (not shown), an exhaust muffler (not shown), and the like. Each of the exhaust pipes  41   R  of the exhaust system  43   R  is tilted in such a manner as to be closer to the central portion of the motorcycle in the width direction since being separated apart downwardly from the right cylinder head  24   R , and is connected to an opening at the outer end of the associated one of the exhaust passages  35   R . 
     The center of the opening at the outer end of each exhaust passage  35   R  is offset forwardly in the longitudinal direction of the motorcycle from a center C R  of an associated one of the combustion chambers  26   R  by the second offset amount L2. 
     The upper ends of the intake valves  36   R  and the exhaust valves  37   R  biased in the valve closing direction by springs are in contact with valve lifters  47   R  supported by the right cylinder head  24   R . Intake side cams  48   R  are in contact with the valve lifters  47   R  associated with the intake valves  36   R  and exhaust side cams  49   R  are in contact with the valve lifters  47   R  associated with the exhaust valves  37   R  The intake side cams  48   R  are provided on a single cam shaft  46   R  which is disposed on the upper side of the plane  38   R . The cam shaft  46   R  is in parallel to the crank shaft  29  and has an axial line perpendicular to the opening/closing operational axial lines of the intake valves  36   R  and the exhaust valves  37   R . In other words, the intake valves  36   R  and the exhaust valves  37   R  are directly opened/closed by the intake side cams  48   R  and the exhaust side cams  49   R  of the cam shaft  46   R , respectively. 
     A plurality (for example, four) of portions, spaced in the axial line direction, of the cam shaft  46   R  are rotatably supported by cam bearing portions  50   R  provided on the right cylinder head  24   R  and a cam holder  51   R  commonly fastened to the cam bearing portions  50   R . Of the four cam bearing portions  50   R , three are each provided on the right cylinder head  24   R  in such a manner as to be disposed between the pair of the intake valves  36   R  and the exhaust valves  37   R  provided for each combustion chamber  26   R , and the remaining cam bearing portion  50   R  is provided on the right cylinder head  24   R  in such a manner as to be located outside the combustion chamber  26   R  disposed at the outermost end on one end side of the cam shaft  46   R  (front end side of the motorcycle). 
     As shown in FIG. 3, the cam shaft  46   R  has oiling holes  53   R  at positions corresponding to the cam bearing portions  50   R . The oiling holes  53   R  are formed in such a manner as to extend from an inside to an outside of the cam shaft  46   R . Lubricating oil is supplied from an oil passage  52   R  formed in the cam shaft  46   R  to the cam bearing portions  50   R  and the cam holder  51   R  via the oiling holes  53   R . Furthermore, an oil groove  54   R  facing to the outer surface of the cam shaft  46   R  is provided in the second cam bearing portion  50   R  from the outermost end on one end side of the cam shaft  46   R , and an oiling passage  55   R  provided in the right cylinder head  24   R  and the right cylinder block  23   R  is in communication with the oil groove  54   R . 
     Each of the intake side cams  48   R  and the exhaust side cams  49   R  has an oiling hole (not shown) in communication with the oil passage  52   R  in the cam shaft  46   R . Lubricating oil is thus also supplied to a slide-contact portion between each of the intake side cams  48   R  and the exhaust side cams  49   R  and an associated one of the valve lifters  47   R  provided for each of the intake valves  36   L  and the exhaust valves  37   L . 
     On the lower side of the plane  38   R , the right cylinder head  24   R  has a plurality (for example, four) of through-holes  56   R  which are spaced in the axial line direction of the cam shaft  46   R . Of the four through-holes  56   R , two are each disposed between adjacent ones of the combustion chambers  26   R . Fastening bolts  57   R  for fastening the right cylinder head  24   R  to the right cylinder block  23   R  are inserted in the through-holes  57   R . 
     Each through-hole  56   R  is adjacent, on one end side (right side in FIG. 9) of the cam shaft  46   R , to an associated one of the exhaust passages  35   R  bypassing the ignition plugs  39   R  provided for the combustion chambers  26   R . The through-hole  56   R  has a positional relationship such that a distance L4 between a center of the through-hole  56   R  and a center C R  of the associated combustion chamber  26   R  is larger than a value L3 (L3&lt;L4). The value L3 is half a distance between the centers C R  of adjacent ones of the combustion chambers  26   R . 
     On the upper side of the plane  38   R , the right cylinder head  24   R  has a plurality (for example, four) of through-holes  58   R  spaced in the axial line direction of the cam shaft  46   R . Of the four through-holes  58   R , two are each disposed between adjacent ones of the combustion chambers  26   R . Fastening bolts  59   R  for fastening the right cylinder head  24   R  to the right cylinder block  23   R  are inserted in the through-holes  58   R . Each through-hole  58   R , that is, fastening bolt  59   R  is disposed at a position where it is partially covered by the cam shaft  46   R . 
     Referring particularly to FIG. 10, of the plurality (for example, four) of the through-holes  58   R , the through-hole  58   R  disposed at the outermost end on one end side of the cam shaft  46   R  is provided in the cam bearing portion  50   R , disposed at the outermost end on the one end side of the cam shaft  46   R , of the four cam bearing portions  50   R . The oil groove  54   R  is provided in the cam bearing portion  50   R  adjacent to the above-described cam bearing portion  50   R  disposed at the outermost end on the one end side of the cam shaft  46   R . 
     Furthermore, a distance L5 between a center of the through-hole  58   R  disposed at the outermost end on the one end side of the cam shaft  46   R  and the center C R  of the combustion chamber  26   R  disposed at the outermost end on the one end side of the cam shaft  46   R  is set to be smaller than the value L3 (L5&lt;L3). The value L3 is, as described above, half the distance between the centers C R  of adjacent ones of the combustion chambers  26   R . 
     A right head cover  60   R  is fastened to the right cylinder head  24   R  in such a manner that a valve system chamber  61   R  for containing the cam shaft  46   R  and the cam holder  51   R  is formed between the right head cover  60   R  and the right cylinder head  24   R . The valve system chamber  61   R  is formed between the right head cover  60   R  and the right cylinder head  24   R  in such a manner as to be offset upwardly from the plane  38   R . 
     A cover portion  62   R  is formed integrally with the right head cover  60   R . Portions of the exhaust pipes  41   R  of the exhaust system  43   R  connected to the exhaust passages  35   R , and the ignition plugs  39   R  disposed downwardly therefrom are covered from the outside by the cover portion  62   R . 
     With respect to the intake passages  34   L  and the exhaust passages  35   L  provided in the left cylinder head  24   L  and the intake passages  34   R  and the exhaust passages  35   R  provided in the right cylinder head  24   R  as described above, the relative positional relationship between the intake passages  34   L  and the exhaust passages  35   L  along the axial line direction of the crank shaft  29  in the left cylinder head  24   L  is set to be nearly equal to the relative positional relationship between the intake passages  34   R  and the exhaust passages  35   R  along the axial line direction of the crank shaft  29  in the right cylinder head  24   R . 
     A throttle body  63 , an intake manifold  64  and an intake system  66  including fuel injection valves  65  provided for each of the combustion chambers  26   L  and  26   R  are disposed over a location between both of the cylinder heads  24   L  and  24   R . The intake manifold  64  is connected to the intake passages  34   L  and  34   R  of both of the cylinder heads  24   L  and  24   R . 
     Secondary air supply passages  44   L  each of which is in communication with the exhaust passage  35   L  are provided in the cylinder head  24   L  and the cylinder block  23   L  of the left engine block B L , and secondary air supply passages  44   R  each of which is in communication with the exhaust passage  35   R  are provided in the cylinder head  24   R  and the cylinder block  23   R  of the right engine block B R . The secondary air supply passages  44   L  are connected to control valves (not shown) via check valves  45  provided in the cylinder block  23   L , and the secondary air supply passages  44   R  are similarly connected to control valves (not shown) via check valves  45  provided in the cylinder block  23   R . 
     Referring particularly to FIG. 11, a transmission mechanism  68   L  is provided between one end portion of the cam shaft  46   L  on the left engine block B L  side and one end portion of the crank shaft  29 . The transmission mechanism  68   L  is adapted to reduce a rotational power of the crank shaft  29  to half and transmit the reduced rotational power to the cam shaft  46   L . A transmission mechanism  68   R  is provided between one end portion of the cam shaft  46   R  on the right engine block B R  side and one end portion of the crank shaft  29 . The transmission mechanism  68   R  is adapted to reduce a rotational power of the crank shaft  29  to half and transmit the reduced rotational power to the cam shaft  46   R . 
     The transmission mechanism  68   L  (or  68   R ) is configured such that an endless chain  71   L  (or  71   R ) is wound around a drive sprocket  69   L  (or  69   R ) fixed on the one end portion of the crank shaft  29  and a driven sprocket  70   L  (or  70   R ) fixed on the one end portion of the cam shaft  46   L  (or  46   R ). As described above, each of the cylinder bores  21   R  constituting the cylinder bore row  22   R  on the right engine block B R  side is offset forwardly in the longitudinal direction of the motorcycle from each of the cylinder bores  21   L  constituting the cylinder bore row  22   L  on the left engine block B L  side by the first offset amount L1. Correspondingly, the transmission mechanism  68   R  on the right engine block B R  side is offset forwardly in the longitudinal direction of the motorcycle from the transmission mechanism  68   L  on the left engine block B L  side. In this case, a gap L6 between both the transmission mechanisms  68   L  and  68   R  is set to be smaller than the first offset amount 1 (L6&lt;L1). 
     A transmission chamber  72   L  for containing the transmission mechanism  68   L  is formed in the front end portion of the left engine block B L  along the longitudinal direction of the motorcycle in such a manner as to extend from the head cover  60   L  to the crank case  27   L  by way of the cylinder head  24   L  and the cylinder block  23   L . 
     To be more specific, one end of the transmission chamber  72   L  faces the valve system chamber  61   L  and the other end thereof faces the crank shaft  29 . Similarly, a transmission chamber  72   R  for containing the transmission mechanism  68   R  is formed in the front end portion of the right engine block B R  along the longitudinal direction of the motorcycle in such a manner as to extend from the head cover  60   R  to the crank case  27   R  by way of the cylinder head  24   R  and the cylinder block  23   R . To be more specific, one end of the transmission chamber  72   R  faces the valve system chamber  61   R  and the other end thereof faces one end of the crank shaft  29 . Accordingly, the other end portions of both the transmission chambers  72   L  and  72   R  are commonly formed in such a manner as to face the one end of the crank shaft  29 . An opening  73  facing to the other end portions of both the transmission chambers  72   L  and  72   R  is provided in the left and right crank cases  27   L  and  27   R , and is covered with a lid member  74  fastened to the left and right crank cases  27   L  and  27   R . 
     In a space on the other end side of the transmission chambers  72   L  and  72   R , a pulse rotor  75  is fixed to the one end portion of the crank case  29  at a position outside both of the sprockets  68   L  and  68   R . A sensor  76  facing to the outer periphery of the pulse rotor  75  is mounted on one of the left and right crank cases  27   L  and  27   R  (left crank case  27   L  in this embodiment). The sensor  76  is adapted to detect the passing of teeth provided on the outer periphery of the pulse rotor  75 . In this way, the rotational position of the crank shaft  29  is detected by the sensor  76 . 
     A pulse rotor  77  is fixed to the one end portion of one of the cam shafts  46   L  and  46   R  (cam shaft  46   L  in this embodiment) at a position outside the driven sprocket  70   L . A sensor (not shown) for detecting the rotational position of the cam shaft  46   L  is mounted to the left cylinder head  24   L  in such a manner as to face the outer periphery of the pulse rotor  77 . 
     The crank shaft  29  is rotated in the rotational direction shown by an arrow  78  in FIG.  11 . At the left side transmission mechanism  68   L , a chain tensioner  79   L  is elastically, slidably in contact with the forward movement portion, i.e., the lower side running portion of the chain  71   L  running counterclockwise from the drive sprocket  69   L  to the driven sprocket  70   L , and a chain guide  80   L  is slidably in contact with the backward movement portion, i.e., the upper side running portion of the chain  71   L  running counterclockwise from the driven sprocket  70   L  to the drive sprocket  69   L . 
     The chain tensioner  79   L  is extended in the running direction of the chain  71   L . One end portion of the chain tensioner  79   L  is turnably supported by the bearing cap  32 , which is closest to the transmission mechanism  68   L , for rotatably supporting the crank shaft  29  in co-operation with the plurality of journal walls  31 , via a supporting shaft  81   L  having an axial line parallel to the rotational axial line of the crank shaft  29 . A tensioner lifter  82   L , which is in contact with an intermediate portion of the chain tensioner  79   L  in the longitudinal direction while pressing the chain tensioner  79   L  onto the chain  71   L , is mounted to the left cylinder block  23   L . 
     The chain guide  80   L  is extended in the running direction of the chain  71   L . One end portion of the chain guide  80   L  is supported via a bolt  83   L  on the journal wall  31  closest to the transmission mechanism  68   L ; and an intermediate portion and the other end portion of the chain guide  80   L  are in contact with and supported by the left cylinder block  23   L  and the left cylinder head  24   L , respectively. 
     At the right side transmission mechanism  68   R , a chain tensioner  79   R  is elastically, slidably in contact with the forward movement portion, i.e., the upper side running portion of the chain  71   R  running counterclockwise from the drive sprocket  69   R  to the driven sprocket  70   R , and a chain guide  80   R  is slidably in contact with the backward movement portion, i.e., the lower side running portion of the chain  71   R  running counterclockwise from the driven sprocket  70   R  to the drive sprocket  69   R . 
     The chain tensioner  79   R  is extended in the running direction of the chain  71   R . One end portion of the chain tensioner  79   R  is tumably supported by the journal wall  31 , which is closest to the transmission mechanisms  68   L  and  68   R , is formed integrally with the left crank case  27   L , via a supporting shaft  81   R  having an axial line parallel to the rotational axial line of the crank shaft  29 . A tensioner lifter  82   R , which is in contact with an intermediate portion of the chain tensioner  79   R  in the longitudinal direction while pressing the chain tensioner  79   R  onto the chain  71   R , is mounted to the right cylinder block  23   R . 
     The chain guide  80   R  is extended in the running direction of the chain  71   R . One end portion of the chain guide  80   R  is supported via a bolt  83   R  on a supporting portion  84  formed integrally with the right crank case  27   R ; and an intermediate portion and the other end portion of the chain guide  80   R  are in contact with and supported by the right cylinder block  23   R  and the right cylinder head  24   R , respectively. 
     One end portion of the transmission chamber  72   L  (or  72   R ) for containing the transmission mechanism  68   L  (or  68   R ) is in communication with the valve system chamber  61   L  (or  61   R ), and the valve system chamber  61   L  (or SI R ) is disposed on the upper side of the plane  38   L  (or  38   R ) containing the axial line of the crank shaft  29  and the axial lines of the cylinder bores  21   L  (or  21   R ). Accordingly, oil supplied from the interior of the valve system chamber  61   L  (or  61   R ) into the one end of the transmission chamber  72   L  (or  72   R ) can be introduced to the other end portion, facing the one end of the crank shaft  29 , of the transmission chamber  72   L  (or  72   R ). A return hole  85  for communicating the bottoms of the other end portions of both of the transmission chambers  72   L  and  72   R  to the crank chamber  28  is provided in the left and right crank cases  27   L  and  27   R . 
     Referring particularly to FIG. 12, a plurality of ribs  88  in contact with and connected to the plurality of journal walls  31  formed integrally with the left crank case  27   L  are formed integrally with the right crank case  27   R  in such a manner as to surround the bearing caps  32 . The return hole  85  is formed in a region extending from the journal wall  31  facing both of the transmission chambers  72   L  and  72   R  to the rib  88  in contact with and connected to the above journal wall  31 . To be more specific, the return hole  85  is composed of a recess  86  provided in the above journal wall  31  in such a manner as to be opened toward the above rib  88  side and a recess  87  provided in the above rib  88  in such a manner as to be opened toward the above journal wall  31  side. 
     The bearing cap  32  is, as described above, fastened to the journal wall  31  with the pair of bolts  33 , and the return hole  85  is extended in the fastening direction of the bearing cap  32  to the journal wall  31 , i.e., the axial line direction of the bolts  33 . 
     The return hole  85  is formed between the crank cases  27   L  and  27   R  in such a manner as to be offset toward the left crank case  27   L  side. To be more specific, of the recesses  86  and  87  constituting the return hole  85 , the recess  86  provided in the journal wall  31  is formed longer in the axial line direction of the bolts  33  than the recess  87  formed in the rib  88 . 
     A transmission case  90  is continued to the left and right engine blocks B L  and B R  in such a manner as to extend downwardly from the crank cases  27   L  and  27   R  and also extend rearwardly in the longitudinal direction of the motorcycle from the cylinder blocks  23   L  and  23   R . In the same manner as the above-described return hole  85 , a passage hole  89  is provided in such a manner as to extend from the bottom of the journal wall  31  disposed between the return hole  85  and the interior of the transmission case  90  to the bottom of the rib  88  in contact with and connected to the journal wall  31 . Accordingly, oil returning from the transmission chambers  72   L  and  72   R  into the crank chamber  28  via the return hole  85  is introduced in the transmission case  90  by way of the passage hole  89 . As described above, oil in the valve system chamber  61   L  and  61   R  is returned to the crank chamber  28  side via the transmission chambers  72   L  and  72   R  on one end sides of the cam shafts  64   L  and  64   R . Since the cam shafts  64   L  and  64   R  are disposed substantially in the horizontal direction, it may be desirable to allow the return of oil from the other end sides of the cam shafts  64   L  and  64   R  to the crank chamber  28  side in the valve system chambers  61   L  and  61   R . To meet the above requirement, a return passage  91   L  (or  91   R ) having one end in communication with the interior of the valve system chamber  61   L  (or  61   R ) on the other end side of the cam shaft  64   L  (or  64   R ) and having the other end in communication with the crank chamber  28  is provided in the left cylinder head  24   L  (or right cylinder head  24   R ) and the left cylinder block  23   L  (or right cylinder block  23   R ). 
     Referring particularly to FIGS. 13 and 14, a water pump  94  including a pump shaft  95  directly connected to the crank case  29  is disposed on the back face of the transmission case  90 . A casing  96  of the water pump  94  is composed of a pump body  97  for rotatably supporting the pump shaft  95 , and a pump cover  98  is fastened to the pump body  97  in such a manner as to cover an impeller  99  fixed to the pump shaft  95 . 
     The pump body  97  is formed integrally with the transmission case  90 . The pump cover  98  is fastened to the pump body  97  with a pump chamber  100  formed between the pump cover  98  and the pump body  97 . The pump shaft  95  is rotatably supported by the pump body  97  in a state where one end thereof projects in the pump chamber  100 . An engagement plate  95   a  to be engaged with an engagement recess  29   a  provided in the other end of the crank shaft  29  is projectingly provided at the other end of the pump shaft  95 . In other words, one end side of the crank shaft  29  is connected to the cam shafts  64   L  and  64   R  via the transmission mechanisms  68   L  and  68   R , while the other end side of the crank shaft  29  is directly connected to the pump shaft  95  of the water pump  94 . 
     The impeller  99  is disposed in the pump chamber  100  and is fixed to the one end of the pump shaft  95 . Over the impeller  99 , a containing portion  101  in communication with the central portion of the pump chamber  100  is formed in the upper portion of the pump cover  98 . 
     A wax type thermostat  102 , which is additionally provided on the water pump  94 , is contained in the containing portion  101  in a state where it is held between the pump body  97  and the pump cover  98 . The thermostat  102  is of a known type, and includes a supporting plate  103  held between the pump body  97  and the pump cover  98 , a thermostat valve  104 , and a bypass valve  105 . 
     A first suction port  106  opened toward one end of the containing portion  101  is provided in the upper portion of the pump body  97  in such a manner as to be openable/closable by the thermostat valve  104 . A second suction port  107  opened toward the other end of the containing portion  101  is provided in the pump cover  98  in such a manner as to be openable/closable by the bypass valve  105 . A discharge port  108  for discharging cooling water discharged depending on rotation of the impeller  99  is provided in the pump cover  98 . The discharge port  108  is in communication with the pump chamber  100 . 
     A water jacket  109   L  (or  109   R ) is provided on the left cylinder block  23   L  (or right cylinder block  23   R ), and a water jacket  110   L  (or  110   R ) in communication with the water jacket  109   L  (or  109   R ) is provided on the cylinder block  23   L  (or  23   R ). The discharge port  108  of the water pump  94  is in communication with the water jackets  109   L  and  109   R  via cooling water supply pipes  111  connected to the left and right cylinder blocks  23   L  and  23   R . 
     A cooling water discharge pipe  112   L  (or  112   R ) for discharge cooling water from the water jackets  110   L  (or  110   R ) is connected to the left cylinder block  24   L  (or right cylinder head  24   R ). The cooling water discharge pipes  112   L  and  112   R  are connected to the second suction port  107  of the water pump  94 , and are also connected to inlets of radiators  113   L  and  113   R , respectively. 
     The radiators  113   L  and  113   R  are disposed over the left and right engine blocks B L  and B R , i.e., both of the cylinder bore rows  22   L  and  22   R . The outlets of both of the radiators  113   L  and  113   R  are connected to the first suction port  106  of the water pump  94 . 
     According to such a cooling water circuit, in a state where the temperature of cooling water is low before the engine is warm, the thermostat  102  closes the thermostat valve  104  and opens the bypass valve  105 . Therefore, cooling water discharged from the discharge port  108  of the water pump  94  is not sucked from the water jackets  109   L ,  110   L ,  109   R  and  110   R  into the water pump  94  by way of the radiators  113   L  and  113   R . On the other hand, as the temperature of cooling water becomes higher along with termination of warming of the engine, the thermostat  102  opens the thermostat valve  104  and closes the bypass valve  105 . Therefore, cooling water discharged from the discharge port  108  of the water pump  94  is sucked from the water jackets  109   L ,  110   L ,  109   R  and  110   R  into the water pump  94  by way of the radiators  113   L  and  113   R  In other words, a bottom bypass type cooling water circuit using the thermostat  102  is formed among the water pump  94 , the water jackets  109   L ,  109   R ,  110   L  and  110   R  and the radiators  113   L  and  113   R    
     A jiggle valve  114  for releasing air in the water pump  94  onto the first suction port  106  side is mounted on the upper portion of the supporting plate  103  of the thermostat  102  disposed over the impeller  99 . 
     Referring particularly to FIG. 13, a main shaft  115  linked with the crank shaft  29 , a counter shaft  116  with a plurality of gear trains capable of being selectively established provided between the main shaft  115  and the counter shaft  116 , and an output shaft  117  linked with the counter shaft  116  via a one-way clutch (not shown) are rotatably supported by the transmission case  90 . Each of the shafts  115 ,  116  and  117  has an axial line parallel to that of the crank shaft  29 . The output shaft  117  for transmitting power to the rear wheel side of the motorcycle projects rearwardly from the back face of the transmission case  90 . 
     A shifter shaft  119  for axially movably supporting a plurality of shifters  118  for selectively establishing the gear trains between the main shaft  115  and the counter shaft  116  is supported by the transmission case  90  at a position below and between the main shaft  115  and the counter shaft  116 . A shift drun  120  for selectively moving one of the shifters  118  is supported by the transmission case  90  at a position adjacent to the shifter shaft  119  in such a manner as to be rotatable on its axis. 
     A motor  121  having a rotational axial line parallel to the axial line of the crank shaft  29  is mounted on the back face of the transmission case  90  at a position above and between the crank shaft  29  and the output shaft  117 . An intermediate shaft  122  is supported by the transmission case  90  at a position between the crank shaft  29  and the motor  121 . A gear train (not shown), which allows transmission of rotational power from the motor  121  to the crank shaft  29  but does not allow transmission of power from the crank shaft  29  to the motor  121 , is provided between the motor  121  and the crank shaft  29  with the intermediate shaft  122  interposed therebetween. Therefore, the power of the motor  121  is transmitted to the crank shaft  29  upon start-up of the engine. 
     A power transmission mechanism  123  actuated upon backward movement is provided between the motor  121  and the output shaft  117 . The mechanism  123  is adapted to transmit rotational power from the motor  121  to the output shaft  117  on the basis of a driver&#39;s operation for backward movement and to rotate the output shaft  117  in a reverse direction upon forward movement. The power transmission mechanism  123  actuated for backward movement cuts off the power transmission from the output shaft  117  to the motor  121  upon operation which is not for backward movement. 
     An electric generator  124  linked with the crank shaft  29  is mounted on the back face of the transmission case  90  in parallel to the axial line of the crank shaft  29 . A clutch  125  coaxial with the main shaft  115 , which is capable of switching the connection/disconnection between the crank shaft  29  and the main shaft  115 , is disposed on the back face of the transmission case  90 . In other words, the electric generator  124  and the clutch  125  are disposed on the back face of the transmission case  90  in parallel to the water pump  94  coaxial with the crank shaft  29 . 
     An oil pump  126  connected to the main shaft  115  via a power transmission mechanism  128  such as a chain is provided in the lower portion of the transmission case  90 . Oil discharged from the oil pump  126  is supplied to respective portions to lubricate the engine main body E via an oil filter  127  (see FIG. 2) provided on the front surface side of the transmission case  90 . The oiling passages  55   L  and  55   R  provided in the left and right cylinder blocks  23   L  and  23   R  and the left and right cylinder heads  24   L  and  24   R  for introducing oil to portions of the cam shafts  46   L  and  46   R  to be lubricated are connected to the oil filter  127 . 
     Referring again to FIGS. 1 and 2, a body frame (not shown) of the motorcycle has steps  130   L  and  130   R  on which the driver&#39;s feet are to rest. The steps  130   L  and  130   R  are mounted on left and right portions positioned behind and below the left and right cylinder heads  24   L  and  24   R  of the engine main body E in such as manner as to project leftwardly and rightwardly therefrom. The inner end of each of the steps  130   L  and  130   R  is offset a distance L7 inwardly in the width direction of the motorcycle from the opening formed at the outer end of each of the exhaust passages  35   L  and  35   R  provided in the cylinder heads  24   L  and  24   R . 
     To prevent the action of the driver&#39;s feet on the steps  130   L  and  130   R  from being obstructed by the left and right cylinder heads  24   L  and  24   R  and the left and right head covers  60   L  and  60   R , the lower rear corners thereof are cut off as shown by reference numeral  131 . 
     The function of this embodiment will now be described. In the horizontally-opposed type multi-cylinder (for example, six cylinder) engine, a pair of left and right cylinder bore rows  22   L  and  22   R  disposed on both sides of the crank shaft  29  extending substantially in the horizontal direction; the left cylinder bore row  22   L  (or right cylinder bore row  22   R ) is composed of a plurality (for example, three) of the cylinder bores  21   L  (or  21   R ) disposed in parallel; and the cam shaft  46   L  (or  46   R ) corresponding to the cylinder bore row  22   L  (or  22   R ) is disposed on an upper side of the plane  38   L  (or  38   R ) containing the axial lines of the cylinder bores  21   L  (or  21   R ) and the axial line of the crank shaft  29  . Accordingly, the valve system mechanism containing the cam shaft  46   L  (or  46   R ) is offset upwardly from the axial lines of the cylinder bores  21   L  (or  21   R ), so that the cylinder head  24   L  (or  24   R ) can be formed in such a manner as to ensure a space under the portion corresponding to the valve system mechanism. In other words, a relatively large space can be ensured under the cylinder head  24   L  (or  24   R ). 
     When the horizontally-opposed type multi-cylinder engine is mounted on a motorcycle in such a manner that the axial line of the crank shaft  29  extends along the longitudinal direction of the motorcycle and the cylinder heads  24   L  and  24   R  project on both sides of the motorcycle in the width direction, it is possible to ensure a sufficient space for allowing the driver&#39;s feet to extend forward at a position under the cylinder heads  24   L  and  24   R  and to set a bank angle a of the motorcycle at a relatively large value. 
     The pairs of the intake valves  36   L  (or  36   R ) and the exhaust valves  37   L  (or  37   R ), each pair being disposed for each cylinder bore  21   L  (or  21   R ), i.e., for each combustion chamber  26   L  (or  26   R ), are disposed in parallel in such a manner as to be offset upwardly from the plane  38   L  (or  38   R ), and are directly opened/closed by the intake side cams  48   L  (or  48   R ) and the exhaust cams  49   L  (or  49   R ) provided on the cam shaft  46   L  (or  46   R ). Accordingly, the valve system mechanism for driving the intake valves  36   L  (or  36   R ) and the exhaust valves  37   L  (or  37   R ) can be significantly simplified. Furthermore, since the cam shafts  46   L  and  46   R  are disposed for the cylinder bore rows  22   L  and  22   R , respectively, the cylinder heads  24   L  and  24   R  can be made compact. 
     Since the intake valves  36   L  (or  36   R ) and the exhaust valves  37   L  (or  37   R ) are disposed in the cylinder head  24   L  (or  24   R ) in such a manner as to be tiled at an acute angle with respect to the plane  38   L  (or  38   R ), it is possible to form the ceiling of each of the combustion chambers  26   L  (or  26   R ) into a pent-roof or semi-spherical shape and hence to set the SNV ratio at a relatively small value. 
     On the opposite side from the disposition side of the intake valves  36   L  ( 36   R ) and the exhaust valves  37   L  (or  37   R ) with respect to the plane  38   L  (or  38   R ), i.e., on the lower side of the plane  38   L  (or  38   R ), the ignition plugs  39   L  ( 39   R ) are mounted to the cylinder head  24   L  (or  24   R ). Each of the ignition plugs  39   L  ( 39   R ) face toward the combustion chamber  26   L  (or  26   R ). Furthermore, in this case, since the intake valves  36   L  (or  36   R ) and the exhaust valves  37   L  (or  37   R ) are tilted at an acute angle with respect to the plane  38   L  (or  38   R ), it is possible to ensure a relatively wide space on the side opposite to the disposition side of the intake valves  36   L  (or  36   R ) and the exhaust valves  37   L  (or  37   R ) with respect to the plane  38   L  (or  38   R ), i.e., the lower side of the plane  38   L  (or  38   R ). Therefore, it is easy to make the ignition plugs  39   L  (or  39   R ) face toward the central portions of the combustion chambers  26   L  (or  26   R ) while avoiding interference with the intake valves  36   L  (or  36   R ) and the exhaust valves  37   L  (or  37   R ) and to increase the degree of freedom of disposition of the ignition plugs  39   L  (or  39   R ). 
     The ignition plugs  39   L  (or  39   R ) are tilted at an acute angle with respect to the plane  38   L  (or  38   R ). With regard to the tilting angle of the ignition plugs  39   L  (or  39   R ), since the intake valves  36   L  (or  36   R ) and the exhaust valves  37   L  (or  37   R ) are tilted at an acute angle with respect to the plane  38   L  (or  38   R ), it is possible to make the ignition plugs  39   L  (or  39   R ) face to the central portions of the combustion chambers  26   L  (or  26   R ) while avoiding the interference with the cam shafts  46   L  (or  46   R ) without setting the tilting angle of the ignition plugs  39   L  (or  39   R ) at a large value. 
     The cylinder head  24   L  (or  24   R ) includes the intake passages  34   L  (or  34   R ) opened toward the side surface of the cylinder head  24   L  (or  24   R ) on the upper side of the plane  38   L  (or  38   R ). Furthermore, the cylinder head  24   L  (or  24   R ) also includes the exhaust passages  35   L  (or  35   R ) opened toward the other side surface of the cylinder head  24   L  (or  24   R ) on the lower side of the plane  38   L  (or  38   R ). In other words, since the intake valves  34   L  (or  34   R ) and the exhaust valves  35   L  (or  35   R ) are provided in such a manner as to be opened toward the side surfaces of the cylinder head  24   L  (or  24   R ) on both sides of the plane  38   L  (or  28   R ), it is easy to connect the intake system  66  and the exhaust system  43   L  (or  43   R ) to the cylinder head  24   L  (or  24   R ). 
     On the projection chart perpendicular to the axial lines of the cylinder bores  21   L  (or  21   R ), the intake passages  34   L  (or  34   R ) are provided in the cylinder head  24   L  (or  24   R ) in such a manner as to cross the plane  38   L  (or  38   R ) substantially at right angles. In other words, since the intake valves  34   L  (or  34   R ) extend substantially in a straight line while being relatively gently curved toward the combustion chambers  26   L  (or  26   R ), it is possible to reduce the intake resistance at the intake passages  34   L  (or  34   R ) and hence to enhance the charging efficiency. 
     The exhaust passages  35   L  (or  35   R ) are provided in the cylinder head  24   L  (or  24   R ) in such a manner as to be curved or swelled to one end side of the cam shaft  46   L  (or  46   R ), i.e., the front side of the motorcycle, in order to bypass the ignition plugs  39   L  (or  39   R ). As a result, the flow resistance in the exhaust passages  35   L  (or  35   R ) is larger than that of the intake passages  34   L  (or  34   R ); however, no problems arise because the exhaust gas from the combustion chambers  26   L  (or  26   R ) is pressurized. 
     Since the cam shaft  46   L  (or  46   R ) is disposed over the axial line of the cylinder bore row  22   L  (or  22   R ) and the exhaust passages  35   L  (or  35   R ) bypass the ignition plugs  39   L  (or  39   R ) by curving toward the front side of the motorcycle, it is easy to ensure space for allowing the driver&#39;s feet to extend forward at a position behind and below the horizontally-opposed type engine mounted on the motorcycle. 
     While the exhaust passages  35   L  (or  35   R ) are downwardly opened toward the lower side surface of the cylinder head  24   L  (or  24   R ), the ignition plugs  39   L  (or  39   R ) are also mounted to the cylinder head  24   L  (or  24   R ) in such a manner as to be tilted downwardly. Accordingly, in the horizontally-opposed type multi-cylinder engine mounted on the motorcycle, it is possible to improve the appearance of the ignition plugs  39   L  (or  39   R ) and the surrounding area, to easily discharge water which has permeated in the vicinity of the ignition plugs  39   L  ( 39   R ) on the outer surface side of the cylinder head  24   L  (or  24   R ), and to easily lay out the exhaust pipes  41   L  ( 41   R ) connected to the exhaust passages  35   L  (or  35   R ). 
     Furthermore, since the cover portion  62   L  (or  62   R ) for covering the ignition plugs  29   L  (or  29   R ) from the outside is formed integrally with the left head cover  60   L  (or right head cover  60   R ) which is connected to the left cylinder head  24   L  (or right cylinder head  24   R ) with the valve system chamber  61   L  (or  61   R ) for containing the cam shaft  46   L  ( 46   R ), it is possible to further improve the appearance of the ignition plugs  39   L  (or  39   R ) and the surrounding area. 
     Since the exhaust passages  35   L  (or  35   R ) are provided-in the cylinder head  24   L  (or  24   R ) in such a manner as to be tilted toward the central side of the motorcycle in the width direction and to be downwardly opened to allow the exhaust pipes  41   L  (or  41   R ) connected to the exhaust passages  35   L  (or  35   R ) to be disposed near the center portion of the motorcycle in the width direction, it is possible to loosen the restriction of the bank angle α of the motorcycle due to the exhaust pipes  41   L  (or  41   R ) and hence to easily ensure the above bank angle α. 
     Furthermore, since the exhaust pipes  41   L  (or  41   R ) are tilted in such a manner that they become closer to the central side of the motorcycle in the width direction, since they are separated apart downwardly from the cylinder head  24   L  (or  24   R ) and are connected to the exhaust passages  35   L  (or  35   R ), it is possible to further loosen the restriction of the bank angle α of the motorcycle due to the exhaust pipes  41   L  (or  41   R ) and hence to more easily ensure the above bank angle a. 
     Since the exhaust valves  37   L  ( 37   R ) are disposed on the upper side of the plane  38   L  (or  38   R ) while the exhaust passages  35   L  (or  35   R ) are opened toward the bottom surface of the cylinder head  24   L  (or  24   R ), it is possible to relatively increase the distance between each of the combustion chambers  26   L  (or  26   R ) and the opening end of an associated one of the exhaust passages  35   L  (or  35   R ) opened toward the bottom surface of the cylinder head  24   L  (or  24   R ). Furthermore, a relatively gentle curving of the exhaust passages  35   L  ( 35   R ) within the plane perpendicular to the axial line of the crank shaft  29  can be made even though the exhaust passages  35   L  (or  35   R ) are opened while being tilted to the central side of the motorcycle in the width direction. This allows suppression of the increase in exhaust resistance. 
     The cover portion  62   L  (or  62   R ) formed integrally with the left head cover  60   L  (right head cover  60   R ) functions to cover connecting portions of the exhaust passages  35   L  (or  35   R ) of the exhaust pipes  41   L  (or  41   R ) from outside. This makes it possible to improve the appearance of the connecting portions of the exhaust passages  35   L  (or  35   R ) of the exhaust pipes  41   L  (or  41   R ). Furthermore, since the exhaust pipes  41   L  (or  41   R ) are separated apart from the cover portion  62   L  (or  62   R ) since directed downwardly, even if the head cover  60   L  (or  60   R ) is made from a synthetic resin, it is possible to avoid occurrence of thermal degradation of the cover portion  62   L  (or  62   R ). 
     With respect to the intake passages  34   L  and the exhaust passages  35   L  provided in the left cylinder head  24   L  and the intake passages  34   R  and the exhaust passages  35   R  provided in the right cylinder head  24   R , the relative positional relationship between the intake passages  34   L  and the exhaust passages  35   L  along the axial line direction of the crank shaft  29  is set to be nearly equal to the relative positional relationship between the intake passages  34   R  and the exhaust passages  35   R  along the axial line direction of the crank shaft  29 . This makes it possible to simplify the structure of the intake system  66  and the exhaust systems  43   L  and  43   R  A plurality of the through-holes  56   L  ( 56   R ) spaced in the axial direction of the cam shaft  46   L  (or  46   R ) are formed in the cylinder head  24   L  (or  24   R ) on the lower side of the plane  38   L  (or  38   R ) to fasten the cylinder head  24   L  (or  24   R ) to the cylinder block  23   L  (or  23   R ). The fastening bolts  57   L  (or  57   R ) are inserted in the through-holes  56   L  (or  56   R ). Furthermore, each through-hole  56   L  (or  56   R ) is adjacent, on one end side of the cam shaft  46   L  (or  46   R ), to an associated one of the exhaust passages  35   L  (or  35   R ) bypassing the ignition plugs  39   L  (or  39   R ) provided in the combustion chambers  26   L  (or  26   R ). The through-hole  56   L  (or  56   R ) has a positional relationship such that a distance L4 between a center of the through-hole  56   L  (or  56   R ) and a center C L  (or C R ) of an associated combustion chamber  26   L  (or  26   R ) is larger than a value L3. The value L3 is half a distance between the centers C L  (or C R ) of adjacent ones of the combustion chambers  26   L  (or  26   R ). This makes it possible to make the curving of the exhaust passages  35   L  (or  35   R ) bypassing the ignition plugs  39   L  (or  39   R ) relatively small. Therefore, the flow resistance of the exhaust passages  35   L  (or  35   R ) are prevented from being excessively increased. 
     On the disposition side of the intake valves  36   R  and the exhaust valves  37   R  with respect to the plane  38   R , the right cylinder head  24   R  has a plurality of the through-holes  58   R  which are spaced in the axial line direction of the cam shaft  46   R . Of the plurality of the through-holes  58   R , the central side through-holes  58   R  are each disposed between adjacent ones of the combustion chambers  26   R . A distance L5 between a center of the through-hole  58   R  disposed at the outermost end on one end side of the cam shaft  46   R  and the center C R  of the combustion chamber  26   R  disposed at the outermost end on the one end side of the cam shaft  46   R  is set to be smaller than the value L3. The value L3 is, as described above, half the distance between the centers C R  of adjacent ones of the combustion chambers  26   R . Accordingly, the end portion of the cylinder head  24   R  on the one end side of the cam shaft  46   R  can be made as close to the center C R  of the combustion chamber  26   R , which is disposed at the outermost end on the curved side of the exhaust passages  35   R  bypassing the ignition plugs  39   R , as possible. This makes the length of the cylinder head  24   R  along the axial direction of the cam shaft  46   R  as small as possible. 
     The cam shaft  46   L  (or  46   R ) is rotatably supported at a plurality of locations spaced in the axial direction of the cam shaft  46   L  (or  46   R ) by the cam bearing portions  50   L  (or  50   R ) provided on the cylinder head  24   L  (or  24   R ) and the cam holder  51   L  (or  51   R ) fastened to the cam bearing portions  50   L  (or  50   R ). The transmission mechanism  68   L  (or  68   R ), which reduces rotational power of the crank shaft  29  to half and transmits the reduced rotational power to the cam shaft  46   L  (or  46   R ), is provided between the crank shaft  29  and the cam shaft  46   L  (or  46   R ). The oil passage  52   L  (or  52   R ), which is capable of supplying oil from the oiling passage  55   L  (or  55   R ) provided in the cylinder head  24   L  (or  24   R ) and the cylinder block  23   L  (or  23   R ), is provided in the cam shaft  46   L  (or  46   R ). On the left cylinder head  24   L  side, oil is supplied from the oil groove  54   L  provided in the cam bearing portion  50   L  disposed at the outermost end on the one end side of the cam shaft  46   L  into the oil passage  52   L  in the cam shaft  46   L  via the oiling hole  53   L  formed in the cam shaft  46   L . On the right cylinder head  24   R  side, the oil groove  54   R  for supplying oil into the oil passage  52   R  in the cam shaft  46   R  via the oiling hole  53   R  formed in the cam shaft  46   R  is formed in the cam bearing portion  50   R  which is provided in the cylinder head  24   R  correspondingly to the combustion chamber  26   R  closest to the transmission mechanism  68   R  among the plurality of combustion chambers  26   R  disposed in the axial direction of the cam shaft  46   R . 
     With this disposition of the oil groove  54   R , it is possible to supply oil into the oil passage  52   R  in the cam shaft  46   R  without restriction of the disposition of the fastening bolts  57   R  and  59   R  for fastening the right cylinder head  24   R  to the right cylinder block  23   R . 
     The cam bearing portion  50   R  closest to the transmission mechanism  68   R  among the plurality of the cam bearing portions  50   R  provided on the right cylinder head  24   R  has the through-hole  58   R  into which the fastening bolt  59   R  among the fastening bolts  57   R  and  59   R  for fastening the cylinder head  24   R  to the cylinder block  23   R  is to be inserted. As a result, the fastening bolt  59   R  between the transmission mechanism  68   R  and the combustion chamber  26   R  is made as close to the combustion chamber  26   R  as possible, so that it is possible to shorten the length of the cylinder head  24   R  along the axial line direction of the cam shaft  46   R . 
     The transmission mechanism  68   R  corresponding to the cam shaft  46   R  on the right cylinder head  24   R  side is offset forwardly along the axial line direction of the crank shaft  29  from the transmission mechanism  68   L  corresponding to the cam shaft  46   L  on the left cylinder head  24   L . In other words, the outermost end on one end side of the cam shaft  46   R  is offset forwardly from that of the cam shaft  46   L , and the transmission mechanism  68   R  is connected to the outermost end on the one end side of the cam shaft  46   R . The above through-hole  58   R  and the above oil groove  54   R  are provided in two of the plurality of the cam bearing portions  50   R  provided on the cam shaft  46   R . Accordingly, it is possible to shorten the length between the transmission mechanism  68   R  and the combustion chamber  26   R  and hence to more effectively shorten the length of the multi-cylinder engine along the axial line direction of the cam shaft  46   L  (or  46   R ). 
     The pair of the cylinder bore rows  22   L  and  22   R  are offset from each other in the axial line direction of the crank shaft  29 . Furthermore, the transmission mechanisms  68   L  and  68   R  are disposed in such a manner that the gap L6 therebetween is smaller than the first offset amount L1 between the cylinder bore rows  22   L  and  22   R . Accordingly, it is possible to set the gap between the transmission mechanisms  68   L  and  68   R  at a smaller value, and hence to decrease the length of the engine main body E along the axial line direction of the cam shaft  46   L  ( 46   R ). 
     Furthermore, since both the transmission mechanisms  68   L  and  68   R  are provided between one end portion of the crank shaft  29  and one end portion of the cam shaft  46   L  and between one end portion of the crank shaft  29  and the one end portion of the cam shaft  46   R , respectively, it is possible to more freely set the gap between the transmission mechanisms  68   L  and  68   R . 
     The outer end opening of each of the exhaust passages  35   L  (or  35   R ) opened toward the bottom surface of the left cylinder head  24   L  (or right cylinder head  24   R ) is offset toward one end side of the cam shaft  46   L  (or  46   R ), i.e., toward the transmission mechanism  68   L  (or  68   R ) from the center C L  (or C R ) of an associated one of the combustion chambers  26   L  (or  26   R ). Accordingly, the exhaust systems  43   L  and  43   R  respectively connected to the exhaust passages  35   L  and  35   R  can be disposed by making effective use of the space between the transmission mechanisms  68   L  and  68   R , so that the entire engine including the exhaust systems  43   L  and  43   R  can be made compact. 
     Since the transmission mechanisms  68   L  and  68   R  are disposed on the front portion of the engine main body E, a relatively large space is formed at a location positioned behind and below the left and right cylinder heads  24   L  and  24   R , the steps  130   L  and  130   R  on which the driver&#39;s feet are to rest can be disposed behind the left and right cylinder heads  24   L  and  24   R  without any difficulty. Furthermore, since the inner end portion of each of the steps  130   L  and  130   R  is offset inwardly from the outer end opening of each of the exhaust passages  35   L  and  35   R  in the width direction of the motorcycle, the projecting amounts of the steps  130   L  and  130   R  in the width direction of the motorcycle is made as small as possible, so that the restriction of the steps  130   L  and  130   R  to the bank angle a can be suppressed. 
     The transmission mechanism  68   L  (or  68   R ) performs power transmission using the chain  71   L  (or  71   R ). The transmission chamber  72   L  ( 72   R ), having one end in communication with the valve system chamber  61   L  (or  61   R ) and the other end facing toward one end of the crank shaft  29  and containing the transmission mechanism  68   L  (or  68   R ), extends from the head cover  60   L  (or  60   R ) to the crank case  27   L  (or  27   R ) via the cylinder head  24   L  (or  24   R ) and the cylinder block  23   L  (or  23   R ). The other end of the transmission chamber  72   L  (or  72   R ) is in communication with the crank chamber  28 . 
     Unlike a belt-type transmission mechanism, the transmission chamber  72   L  (or  72   R ) containing the transmission mechanism  68   L  (or  68   R ) allows oil to flow therethrough. Accordingly, it is possible to eliminate the necessity of provision of any means for preventing leakage of oil from the crank case  27   L  (or  27   R ) side onto the transmission chamber  72   L  (or  72   R ) side. More specifically, the necessity of provision of a seal structure on the crank case  27   L  (or  27   R ) is eliminated. Therefore, the engine is made as compact as possible. 
     Furthermore, since the cam shaft  46   L  (or  46   R ) is disposed over the crank shaft  29 , oil in the valve system  61   L  (or  61   R ) is allowed to flow onto the crank shaft  29  side at the lower level through the transmission chamber  72   L  (or  72   R ). As a result, oil in the valve system chamber  61   L  (or SI R ) is easily returned to the crank case  27   L  (or  27   R ) side. 
     In addition, the return hole  85  is provided in the left and right crank cases  27   L  and  27   R  to communicate the bottom portions of the other ends of the transmission chambers  72   L  and  72   R  into the crank chamber  28 . Accordingly, it is not required to provide oil return passages specialized for the cylinder blocks  23   L  and  23   R  and the cylinder heads  24   L  and  24   R  for returning oil from at least the transmission chambers  72   L  and  72   R  into the crank chambers  28 . Therefore, the cylinder blocks  23   L  and  23   R  and the cylinder blocks  24   L  and  24   R  can be made compact and reduced in weight. 
     The crank shaft  29  is rotatably supported by a plurality of the journal walls  31  formed integrally with the left crank case  27   L  and a plurality of bearing caps  32  fastened to the journal walls  31 . The return hole  85  is extended in the fastening direction of the bearing caps  32  to the journal walls  31 . Accordingly, it is possible to make the opening area of the return hole  85  relatively wide without reducing the supporting rigidity of the crank shaft  29 . Therefore, the return of oil into the crank chamber  28  is enhanced. 
     The return hole  35  is formed in the left and right crank cases  27   L  and  27   R  in such a manner as to be offset toward the left crank case  27   L  side. Accordingly, it is possible to increase the opening area of the return hole  85  avoiding a reduction in rigidity of the crank case on which the journal walls  31  are not integrally formed, i.e., the right crank case  27   R . Therefore, the return of the oil is further enhanced. 
     In the transmission mechanism  68   L  provided between the left side cam shaft  46   L  and the crank shaft  29 , the chain tensioner  79   L  extending along the running direction of the chain  71   L  is elastically, slidably in contact with the chain  71   L . One end of the chain tensioner  79   L  in the longitudinal direction is turnably supported by the bearing cap  32  closest to the transmission mechanism  68   L  among a plurality of the bearing caps  32 . With this configuration, it is possible to moderate the restriction in the rotatably supporting position of the chain tensioner  79   L  and to confine the behavior of the chain  71   L  by setting the length of the chain tensioner  79   L  at a relatively large value. 
     Since the transmission mechanism  68   L  is provided between one end portion of the cam shaft  46   L  and one end portion of the crank shaft  29 , it is not required to take into account the disposition of the rotatably supporting portion of the chain tensioner  79   L  at a position where the chain tensioner  79   L  does not interfere with a crank weight of the crank shaft  29 . This makes it possible to simply set the rotatably supporting position of the chain tensioner  79   L . 
     Since one end of the chain tensioner  79   L  for the transmission mechanism  68   L  on the cylinder block  23   L  side on which the journal walls  31  are integrally formed is rotatably supported by the bearing cap  32  closest to the transmission mechanism  68   L , it is possible to simply set the rotatably supporting position of the chain tensioner  79   L  by making effective use of one of the bearing caps  32  necessarily provided for the horizontally-opposed type multi-cylinder engine. 
     The pump shaft  95  of the water pump  94  is directly connected to the other end of the crank shaft  29  with one end side connected to the transmission mechanisms  68   L  and  68   R , i.e., the rear end of the crank shaft  29  along the longitudinal direction of the motorcycle, and the water pump  94  is directly driven by the crank shaft  29 . Accordingly, it is possible to eliminate the necessity of a gear, a chain, a belt, etc. required for driving the conventional water pump, and therefore simplify the drive mechanism of the water pump  94 . 
     The pulse rotor  75  for detecting a rotational position of the crank shaft  29  is mixed to one end portion of the crank shaft  29 . By use of the pulse rotor  75 , it is possible to easily detect a rotational position of the crank shaft  29  with no obstruction by the water pump  94 . 
     Since the water pump  94  is disposed on the rear side in the longitudinal direction of the motorcycle, a piping system for cooling water, connected to the water pump  94 , can be disposed at an inconspicuous position. 
     Since the radiators  113   L  and  113   R  are respectively disposed over the engine blocks B L  and B R , i.e., over the cylinder bore rows  22   L  and  22   R , pipes for cooling water between the engine and the radiators  113   L  and  113   R  are made nearly equal on the left and right sides or are even shortened. 
     Since the electric generator  124  and the clutch  125  are disposed in parallel with the water pump  94 , it is not required to increase the length of the crank shaft  29  for disposing the electric generator  124  and the clutch  125  in spite of the fact that the water pump  94  is directly driven by the crank shaft  29 . Accordingly, it is possible to make the engine compact in the axial direction of the crank shaft  29 . 
     The casing  96  of the water pump  94  is composed of the pump body  97  for rotatably supporting the pump shaft  95 , and the pump cover  98  connected to the pump body  97  in such a manner as to cover the impeller  99  fixed to the pump shaft  95 . The thermostat  102  held between the pump body  97  and the pump cover  98  is contained in the containing portion  101  formed in the pump cover  98 . As a result, in the case of additionally providing the thermostat  102  in the water pump  94 , it is possible to reduce the number of parts, and hence to reduce the cost and weight and the number of assembling steps. 
     The first suction port  106  opened toward one end of the containing portion  101  is provided in the pump body  97  in such a manner as to be in communication with the radiators  113   L  and  113   R . The second suction port  107  opened toward the other end of the containing portion  101  for introducing water from the engine not by way of the radiators  113   L  and  113   R  is provided in the pump cover  98 . The thermostat  102  having the thermostat valve  104  for opening/closing the first suction port  106  and the bypass valve  105  for opening/closing the second suction port  107  is contained in the containing portion  101 . Accordingly, when the temperature of cooling water is low, the thermostat valve  104  is closed and the bypass valve  105  is opened, while as the temperature of cooling water is increased, the thermostat valve  104  is opened and the bypass valve  105  is closed. In this way, the bottom-bypass type cooling water circuit can be simply obtained. 
     Since the discharge port  108  for discharging cooling water discharged depending on rotation of the impeller  99  is provided in the pump cover  98 , it is possible to simply obtain a circuit for introducing cooling water from the water pump  94 . 
     Since the thermostat  102  is disposed over the impeller  99 , it is possible to release air in the water pump  94  by means of the jiggle valve  114  of the thermostat  102 . 
     According to the first aspect of the present invention, oil is returned to the crank chamber from the valve chamber via a return hole so that oil circulates in the transmission chamber housing the transmission mechanisms. As it is therefore no longer necessary to provide a dedicated oil returning oil passage at the cylinder blocks and the cylinder heads for at least the flow of oil returning to the crank chamber from the transmission chamber, the cylinder blocks and cylinder heads can be made smaller accordingly and can also be made more lightweight. 
     According to the second aspect of the present invention, the surface area of the opening for the return hole can be set to be relatively large and the extent to which oil is returned to the crank chamber can be improved without detriment to the rigidity with which the crankshaft is supported. 
     According to the third aspect of the present invention, in a horizontally opposed engine where pairs of cylinder bore rows oppose each other in a substantially horizontal plane, lowering of the rigidity of a crankcase on a side where a journal wall supporting the crankshaft together with bearing caps is not integrally formed is avoided, the surface area of the return hole can be made larger and the extent to which oil is returned to the crank chamber is dramatically improved. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.