embodiments of the present invention will be described below with reference to the accompanying drawings . fig1 through 10 show a first embodiment wherein the present invention is applied to a four - cycle horizontally opposite four - cylinder engine . in fig1 through 3 , the four - cycle horizontally opposite four - cylinder engine is mounted on an airplane , for example . the four - cycle horizontally opposite four - cylinder engine is housed in a front cowl of the airframe of the airplane with the axis of a crankshaft 11 extending longitudinally of the airplane , and a spinner having a plurality of propellers is coaxially coupled to the crankshaft 11 . as also shown in fig4 , the engine has an engine body 12 including a left engine block 13 l disposed on the left side of the engine as viewed from behind and a right engine block 13 r disposed on the right side of the engine as viewed from behind . the left engine block 13 l includes a left crankcase 14 l and a left cylinder block 15 l coupled to the left crankcase 14 l . the right engine block 13 r includes a right crankcase 14 r coupled to the left crankcase 14 l and a right cylinder block 15 r coupled to the right crankcase 14 r remotely from the left crankcase 14 l . the left cylinder block 15 l includes a left cylinder barrel 16 l coupled to the left crankcase 14 l and a left cylinder head 17 l integrally formed with the left cylinder barrel 16 l remotely from the left crankcase 14 l . the right cylinder block 15 r includes a right cylinder barrel 16 r coupled to the right crankcase 14 r and a right cylinder head 17 r integrally formed with the right cylinder barrel 16 r remotely from the right crankcase 14 r . as also shown in fig5 and 6 , the cylinder barrels 16 l , 16 r of the cylinder blocks 15 l , 15 r have respective pairs of cylinder bores 18 l , 18 l ; 18 r , 18 r disposed on both sides of the crankshaft 11 and confronting each other . the cylinder bores 18 l , 18 l ; 18 r , 18 r are arrayed in the axial direction of the crankshaft 11 and offset with respect to each other in the axial direction of the crankshaft 11 . pistons 20 l . . . , 20 r . . . , which define combustion chambers 19 l . . . , 19 r . . . between the pistons and the cylinder heads 17 l , 17 r , are slidably fitted in the respective cylinder bores 18 l . . . , 18 r . . . . the engine blocks 13 l , 13 r are arranged in opposed relation to each other with the axes of the cylinder bores 18 l . . . , 18 r . . . being disposed substantially horizontally . the left and right crankcases 14 l , 14 r are fastened to each other to jointly make up a crankcase 21 . the crankshaft 11 , connected to the pistons 20 l . . . , 20 r . . . by connecting rods 22 l . . . , 22 r . . . , is rotatably supported between the left and right crankcases 14 l , 14 r . the left crankcase 14 l has a front journal support wall 23 l , a first intermediate journal support wall 24 l , a second intermediate journal support wall 25 l , a third intermediate journal support wall 26 l and a rear journal support wall 27 l . the walls support a left half of the crankshaft 11 on both the front and rear sides of the connecting rods 22 l . . . and are longitudinally spaced from each other . the right crankcase 14 r has a front journal support wall 23 r , a first intermediate journal support wall 24 r , a second intermediate journal support wall 25 r , a third intermediate journal support wall 26 r and a rear journal support wall 27 r . the walls support a right half of the crankshaft 11 on both the front and rear sides of the connecting rods 22 r . . . and are longitudinally spaced from each other . the crankshaft 11 is rotatably supported by the journal support walls 23 l through 27 l of the left crankcase 14 l and the journal support walls 23 r through 27 r of the right crankcase 14 r . the journal support walls 23 l through 27 l and 23 r through 27 r of the left and right crankcases 14 l , 14 r are fastened by pairs of stud bolts 28 . . . and nuts 29 . . . , which extend vertically across the crankshaft 11 . the stud bolts 28 . . . for fastening the front journal support walls 23 l , 233 r and the rear journal support walls 27 l , 24 r are longer than the stud bolts 28 . . . for fastening the first , second , and third intermediate journal support walls 24 l through 26 l ; 24 r through 26 r . the nuts 29 . . . engage an outer surface of the right crankcase 14 r and are threaded over the stud bolts 28 . . . which are mounted on the front journal support wall 23 l of the left crankcase 14 l and inserted through the front journal support wall 23 r of the right crankcase 14 r . the nuts 29 . . . engage an outer surface of the left crankcase 14 l and are threaded over the stud bolts 28 . . . which are mounted on the rear journal support wall 27 r of the right crankcase 14 r and inserted through the rear journal support wall 27 l of the left crankcase 14 l . the nuts 29 . . . are threaded over the stud bolts 28 . . . that are mounted on the second and third intermediate journal support walls 25 l , 26 l of the left crankcase 14 l and are inserted through the second and third intermediate journal support walls 25 r , 26 r of the right crankcase 14 r . the nuts 29 . . . are held in engagement with the second and third intermediate journal support walls 25 r , 26 r . the nuts 29 . . . are threaded over the stud bolts 28 . . . that are mounted on the first intermediate journal support wall 24 r of the right crankcase 14 r and inserted through the first intermediate journal support wall 24 l of the left crankcase 14 l . the nuts 29 . . . are held in engagement with the first intermediate journal support walls 24 l . the left and right engine blocks 13 l , 13 r are coupled to each other by pairs of through bolts 30 . . . and pairs of two sets of stud bolts 32 . . . that are disposed in portions corresponding to the first , second , and third intermediate journal support walls 24 l through 26 l and 24 r through 26 r of the crankcases 14 l , 14 r . the through bolts 30 . . . extend through the left and right engine blocks 13 l , 13 r in such a manner to sandwich , between themselves and the crankshaft 11 , the pairs of stud bolts 28 . . . . the stud bolts 28 are disposed on the first , second , and third intermediate journal support walls 24 l through 26 l and 24 r through 26 r in order to fasten the support walls 24 l through 26 l and 24 r through 26 r to each other . nuts 31 . . . are threaded over the opposite ends of the through bolts 30 . . . , which project from the cylinder heads 17 l , 17 r of the left and right engine blocks 13 l , 13 r . in order to prevent the through bolts 30 . . . from rotating when the nuts 31 . . . are tightened , hexagonal tool engaging portions 30 a for engagement with a tool ( not shown ) are coaxially disposed on the opposite ends of the respective through bolts 30 . . . so as to project from the nuts 31 . . . . of the two sets of stud bolts 32 . . . , one set of stud bolts 32 . . . is mounted on the front journal support wall 23 l of the left crankcase 13 l and extends through the right engine block 13 r and nuts 33 . . . are threaded over the stud bolts 32 . . . , which project from the cylinder head 17 r of the right engine block 13 r . of the two sets of stud bolts 32 . . . , the other set of stud bolts 32 . . . is mounted on the rear journal support wall 27 r of the right crankcase 13 r and extends through the left engine block 13 l , and nuts 33 . . . are threaded over the stud bolts 32 . . . which project from the cylinder head 17 l of the left engine block 13 l . the stud bolts 32 . . . are disposed in positions for sandwiching , between themselves and the crankshaft 11 , the pair of stud bolts 28 . . . fastening the front journal support walls 23 l , 23 r of the left and right crankcases 13 l , 13 r and the pair of stud bolts 28 . . . fastening the rear journal support walls 27 l , 27 r of the left and right crankcases 13 l , 13 r . as shown in fig7 , the through bolts 30 . . . and the stud bolts 32 . . . are disposed in a surrounding relation to the cylinder bores 18 l . . . , 18 r . . . at 90 °- spaced intervals . the cylinder blocks 13 l , 13 r have a plurality of integral mounting bosses 34 . . . for the through bolts 30 . . . and the stud bolts 32 . . . to extend therethrough . the mounting bosses 34 . . . extend from the surfaces of the cylinder barrels 16 l , 16 r , which are attached to the crankcase 21 , to the cylinder heads 17 l , 17 r and surround the cylinder bores 18 l . . . , 18 r . . . . joint walls 35 . . . are integrally mounted on the cylinder blocks 13 l , 13 r as shown in fig8 . the joint walls 35 . . . join at least one of the two adjacent sets of the mounting bosses 34 , 34 , which are disposed on corresponding portions between the mutually adjacent cylinder bores 18 l , 18 l ; 18 r , 18 r arrayed in the axial direction of the crankshaft 11 . the pair of mutually adjacent mounting bosses 34 , 34 are on upper walls of the cylinder blocks 13 l , 13 r in the first embodiment . a support tube 38 , which is jointly made up of the left and right crankcases 14 l , 14 r , is formed so as to project forwardly on a front portion of the crankcase 21 . the crankshaft 11 has a front portion extending coaxially through the support tube 38 and projecting from the front end of the support tube 38 . a ring gear 39 is fixed to the portion of the crankshaft 11 , which projects from the front end of the support tube 38 . the spinner ( not shown ) is coaxially mounted on the ring gear 39 . a slide bearing 40 is interposed between the front portion of the support tube 38 and the crankshaft 11 , and an annular seal member ( not shown ) is interposed between the support tube 38 and the crankshaft 11 forwardly of the slide bearing 40 . for starting the engine , a starter 41 applies a rotational drive force to the crankshaft 11 . the starter 41 includes a starter motor 42 and a pinion 43 . the motor 42 is supported on a lower portion of the left crankcase 14 of the crankcase 21 . the pinion 43 projects into mesh with the ring gear 39 when the rotational speed of the starter motor 42 becomes a predetermined value or higher . after the engine has started to operate , the pinion 43 is released out of mesh with the ring gear 39 back into its original position . the crankshaft 11 has a plurality of circumferentially spaced teeth 44 within the support tube 38 . a pair of crankshaft angle sensors 45 , 45 , for detecting a crankshaft angle , is mounted on the support tube 38 by the projections 44 . . . in 180 °- spaced relation to each other . as also shown in fig9 and 10 , a water pump 46 , which can be rotated by the crankshaft 11 , is mounted on an end of the crankcase 21 along the axis of the crankshaft 11 , i . e ., a rear end of the crankcase 21 in the first embodiment . a drive gear 47 is coaxially mounted on a rear end of the crankshaft 11 , which projects from the rear journal support walls 27 l , 27 r . a rotor 49 of a generator 48 , which is mounted in a rear portion of the crankcase 21 , is coaxially and relatively immovably connected to the drive gear 47 . a cover 51 is mounted on the rear end of the crankcase 21 . the generator 48 has a stator 50 mounted on the cover 51 . the water pump 46 has a pump housing 54 including a case 52 , which integrally has a cylindrical shaft support 52 a that is fitted in the cover 51 in a light - tight manner and a pump cover 53 sandwiching the case 52 between itself and the cover 51 . the case 52 and the pump cover 53 are fastened together to the cover 51 . a pump shaft 55 , which extends through the shaft support 52 a in a light - tight manner , is rotatably supported by the shaft support 52 a . an end of the pump shaft 55 , which projects from the shaft support 52 a , is rotatably supported by the crankcase 21 . rotary vanes 57 are fixed to the other end of the pump shaft 55 within a pump chamber 56 that is defined in the pump housing 54 . a driven gear 58 , which is fixed to the pump shaft 55 between the shaft support 52 a and the crankcase 21 , is held in mesh with an idle gear 59 that is rotatably supported between the crankcase 21 and the cover 51 . the idle gear 59 is in mesh with the drive gear 47 . cooling cylinder water jackets 60 l , 60 r are disposed in the respective cylinder barrels 16 l , 16 r . head water jackets 61 l , 61 r communicating respectively with the cylinder water jackets 60 l , 60 r are disposed in the respective cylinder heads 17 l , 17 r , which are integrally formed with the cylinder barrels 16 l , 16 r . the water pump 46 serves to circulate a coolant between the cylinder and head water jackets 60 l , 60 r ; 61 l , 61 r . the crankcase 21 has coolant supply passages 62 l , 62 r for guiding the coolant from the water pump 46 and coolant return passages 63 l , 63 r for guiding the coolant that is delivered out of the cylinder water jackets 60 l , 60 r . the coolant supply passages 62 l , 62 r and the coolant return passages 63 l , 63 r are disposed parallel to the axis of the crankshaft 11 on both sides of the axes of the cylinder bores 18 l . . . , 18 r . . . . the cylinder and head water jackets 60 l , 60 r ; 61 l , 61 r are formed such that the coolant supplied from the coolant supply passages 62 l , 62 r returns from the cylinder water jackets 60 l , 60 r via the head water jackets 61 l , 61 r to the cylinder water jackets 60 l , 60 r . the cylinder water jackets 60 l , 60 r are divided into supply jacket portions 64 . . . and return jacket portions 65 . . . . the supply jacket portions 64 . . . communicate with the coolant supply passages 62 l , 62 r and also with the head water jackets 61 l , 61 r . the return jacket portions 65 . . . communicate with the head water jackets 61 l , 61 r at positions spaced from the supply jacket portions 64 . . . and are defined in the cylinder barrels 16 l , 16 r . the cover 51 joined to the crankcase 21 has a passage 66 for guiding the coolant discharged from the pump chamber 56 of the water pump 46 to the coolant supply passages 62 l , 62 r . the supply jacket portions 64 and the return jacket portions 65 . . . , with superposed portions surrounding substantial half of the cylinder bores 18 l . . . , 18 r . . . , are formed in the cylinder barrels 16 l , 16 r . a plurality of supply and return branch passages 67 l , 67 r ; 68 l , 68 r are disposed in the crankcase 21 and the cylinder barrels 16 l , 16 r . the supply and return branch passages 67 l , 67 r ; 68 l , 68 r are connect the portions of the supply jacket portions 64 . . . and the return jacket portions 65 . . . , which correspond to the cylinder bores 18 l . . . , 18 r . . . , to the coolant supply passages 62 l , 62 r and the coolant return passages 63 l , 63 r . the coolant supply passages 62 l , 62 r are reduced in diameter stepwise in a direction away from the water pump 46 . the inside diameter of the supply passages 62 l , 62 r in the cylinder bores 18 l , 18 r that is most remote from the water pump 46 is smaller than the inside diameter thereof in the cylinder bores 18 l , 18 r closer to the water pump 46 . when the cylinder blocks 13 l , 13 r are cast , the cylinder water jackets 60 l , 60 r are formed within the cylinder barrels 16 l , 16 r as ring - shaped cavities surrounding the respective cylinder bores 18 l . . . , 18 r . . . . rod members 69 . . . are fitted into the cylinder barrels 16 l , 16 r from the cylinder heads 17 l , 17 r so as to lie on a straight line interconnecting the axes of the cylinder bores 18 l . . . , 18 r . . . . the rod members 69 . . . divide the cavities halfway into the supply jacket portions 64 . . . and the return jacket portions 65 . . . . the rod members 69 . . . have intermediate portions smaller in diameter for forming an annular passage 70 for removing air from the supply jacket portions 64 . . . into the return jacket portions 65 . . . , between themselves and the cylinderbarrels 16 l , 16 r . plug members 105 . . . , 106 . . . , for preventing the rod members 69 . . . from being released from . the cylinder heads 17 l , 17 r , are threaded in the cylinder heads 17 l , 17 r in abutment against the rod members 69 . . . . referring to fig9 in particular , the coolant return passages 63 l , 63 r communicate with each other through a joint pipe 71 . a first return conduit 72 communicates with the coolant return passage 63 l and is connected to the crankcase 21 for guiding the coolant to a radiator ( not shown ). the coolant , which returns from the radiator , is guided to a second return conduit 74 . a thermostat 73 for guiding the coolant from the first return conduit 72 to the second return conduit 74 by bypassing the radiator when the temperature of the coolant is low is disposed between the first and second return conduits 72 , 74 . the second return conduit 74 is connected to a return joint pipe 75 that is joined to the pump cover 53 of the water pump 46 . a steam conduit 76 , for guiding a steam evaporated by heating into an expansion tank 77 , is connected to an upper portion of the first return conduit 72 . a third return pipe 78 for guiding the coolant that is condensed in the expansion tank 77 is connected to a return joint pipe 79 that is joined to the pump cover 53 of the water pump 46 . an oil filter 81 is disposed laterally of an oil pan 80 mounted on a lower portion of the crankcase 21 . the oil filter 81 houses therein an oil cooler that is supplied with the coolant from the water pump 46 via a supply conduit 82 . the coolant from the oil cooler is returned to the water pump 46 via a return conduit 83 . intake ports 85 l . . . , 85 r . . . corresponding individually to the combustion chambers 9 l . . . , 19 r . . . are defined in upper portions of the left and right cylinder heads 17 l , 17 r . the intake ports 85 l . . . , 85 r . . . are bifurcated and communicate with the combustion chambers 19 l . . . , 19 r . . . . arcuately curved intake pipes 86 l . . . , 86 r . . . are connected respectively to the intake ports 85 l . . . , 85 r . . . . electromagnetic fuel injector valves 87 l . . . , 87 r . . . for injecting a fuel into the intake ports 85 l . . . , 85 r . . . are mounted respectively in intermediate portions of the intake pipes 86 l . . . , 86 r . . . . the electromagnetic fuel injector valves 87 l . . . in the left engine block 13 l are connected to a common fuel rail 88 l , and the electromagnetic fuel injector valves 87 r . . . in the right engine block 13 r are connected to a common fuel rail 88 r . an intake chamber 89 is disposed above the crankcase 21 of the engine body 12 and supported by the engine body 12 . the intake pipes 86 l . . . , 86 r . . . have upstream ends connected to downstream ends of joint pipes 90 l . . . , 90 r . . . , which have upstream ends projecting into the intake chamber 89 from both sides thereof . in the intake chamber 89 , the upstream ends of the joint pipes 90 l 90 r . . . are spread into a flaring shape and open rearwardly . throttle bodies 92 , 92 each having a throttle valve 91 angularly movably supported therein with downstream ends juxtaposed and connected to a rear portion of the intake chamber 89 . air cleaners 93 , 93 are connected respectively to upstream ends of the throttle bodies 92 , 92 . the air cleaners 93 , 93 are supported on support stays 94 , 94 , which are mounted on the intake chamber 89 and extend rearwardly . exhaust ports 95 l . . . , 95 r . . . , which correspond individually to the combustion chambers 19 l . . . , 19 r . . . , are defined in lower portions of the left and right cylinder heads 17 l , 17 r . exhaust pipes 96 l . . . , 96 r . . . extending below the engine body 12 and rearwardly are connected respectively to the exhaust ports 95 l . . . , 95 r . . . . substantially h - shaped head covers 97 l , 97 r are joined respectively to the left and right cylinder heads 17 l , 17 r . valve operating devices ( not shown ) for actuating intake valves and exhaust valves to control the introduction of intake air into the combustion chambers 19 l . . . , 19 r . . . and the discharge of exhaust gases from the combustion chambers 19 l . . . , 19 r . . . are disposed between the head covers 97 l , 97 r and the cylinder heads 17 l , 17 r . covers 98 l , 98 r are fastened to upper portions of the head covers 97 l . . . . the covers 98 l , 98 r cover intake valve operating portions of the valve operating devices . covers 99 l , 99 r , which cover exhaust valve operating portions of the valve operating devices , are fastened to lower portions of the head covers 97 l . . . . the intake valve operating portions of the valve operating devices , which are disposed between the head covers 97 l , 97 r and the cylinder heads 17 l , 17 r , produce valve opening drive forces with push rods that are pushed upwardly in the intake stroke by the power transmitted from the drive gear 47 of the crankshaft 11 . the push rods associated with the respective combustion chambers 19 l . . . , 19 r . . . are axially movably inserted in rod guide tubes 100 l , 100 r . the tubes 100 l , 100 r are disposed below the cylinder blocks 15 l , 15 r on the left and right sides of the crankcase 21 and interconnecting longitudinally central portions of the lower portions of the left and right crankcases 14 l , 14 r and the head covers 97 l , 97 r . the exhaust valve operating portions of the valve operating devices , which are disposed between the head covers 97 l , 97 r and the cylinder heads 17 l , 17 r , produce valve opening drive forces with pull rods that are pulled downwardly in the exhaust stroke by the power transmitted from the drive gear 47 of the crankshaft 11 . the pull rods associated with the respective combustion chambers 19 l . . . , 19 r . . . are axially movably inserted in rod guide tubes 101 l , 101 r . the tubes 101 l , 101 r are disposed below the rod guide tubes 100 l , 100 r and for interconnecting the longitudinally central portions of the lower portions of the left and right crankcases 14 l , 14 r and the head covers 97 l , 97 r . thus , the rod guide tubes 100 l , 100 r , 101 l . . . , 101 r . . . are disposed to interconnect the longitudinally central portions of the lower portions of the left and right crankcases 14 l , 14 r and the head covers 97 l , 97 r . the pair of mutually adjacent mounting bosses 34 , 34 on the upper wall of the cylinder blocks 13 l , 13 r are connected by the joint walls 35 . . . and are integral with the cylinder blocks 13 l , 13 r . the rod guide tubes 100 l , 100 r , 101 l . . . , 101 r . . . and the joint walls 35 are disposed on upper and lower sides of the cylinder blocks 13 l , 13 r . pairs of ignition plugs 102 l , 102 l . . . , 102 r , 102 r . . . , which are associated with the respective combustion chambers 19 l . . . , 19 r . . . , are mounted in the cylinder heads 17 l , 17 r . ignition coils 103 l . . . , 103 r . . . as electric accessories are mounted on upper side surfaces of the cylinder . heads 17 l , 17 r between the intake pipes 86 l , 86 l ; 86 r , 86 r . the ignition coils 103 l 103 r . . . are disposed by a pair on each side of the intake chamber 89 . pairs of high - tension cords 104 . . . connected to the ignition coils 103 l . . . , 103 r . . . are connected to the ignition plugs 102 l , 102 l . . . , 102 r , 102 r . . . . to allow the fuel to be reliably ignited in the combustion chambers 19 l . . . , 19 r . . . even in the event that one of the ignition coils 103 l . . . , 103 r . . . malfunctions , a pair of high - tension cords 104 , 104 connected to the same ignition coils 103 l . . . , 103 r . . . is connected to the ignition plugs 102 l . . . , 102 r . . . of the different combustion chambers 19 l . . . , 9 r . . . . an electronic control unit 105 ′ for controlling the operation of the engine is mounted on the outer surface of a front side wall of the intake chamber 89 . an intake pressure sensor 106 ′ and an intake temperature sensor 107 are inserted from the electronic control unit 105 ′ into the intake chamber 89 through the front side wall of the intake chamber 89 . the intake pressure sensor 106 ′ and an intake temperature sensor 107 is for detecting the intake pressure and temperature , respectively , in the intake chamber 89 . the electromagnetic fuel injector valves 87 l . . . , 87 r . . . , the ignition coils 103 l . . . , 103 r . . . , and the electronic control unit 105 ′ are disposed around the intake chamber 89 . the electromagnetic fuel injector valves 87 l . . . , 87 r . . . , the ignition coils 103 l . . . 103 r . . . and the electronic control unit 105 ′ are covered with a shield cover 108 , which is mounted on the engine body 12 in a covering relationship to at least a portion of the intake chamber 89 . in the first embodiment , the shield cover 108 is made of a steel sheet , for example , in a covering relationship to a substantial portion of the intake chamber 89 except a rear portion thereof and an upper portion of the engine body 12 . the shield cover 108 has an opening edge formed in contact with the engine body 12 . portions of the high - tension cords 104 extending from the ignition coils 103 l . . . 103 r . . . are also covered with the shield cover 108 . since the electromagnetic fuel injector valves 87 l . . . , 87 r . . . , the ignition coils 103 l . . . , 103 r . . . , and the electronic control unit 105 ′ are covered with the single shield cover 108 , the electric accessories can be shielded . the number of parts used is reduced and the overall engine is made more compact than if the electric accessories are individually shielded . as the portions of the high - tension cords 104 . . . are covered with the shield cover 108 , those portions of the shield cover 108 may have their individual shields removed . therefore , a secondary voltage drop across the high - tension cords 104 . . . may be improved by removing the individual shields . thus , the electronic control unit 105 ′ is mounted on the outer surface of the front side wall of the intake chamber 89 . further , the intake pressure sensor 106 ′ and the intake temperature sensor 107 for detecting the intake pressure and temperature , respectively , in the intake chamber 89 are inserted from the electronic control unit 105 ′ into the intake chamber 89 through the front side wall of the intake chamber 89 . the electronic control unit 105 ′ can be shielded , and also the intake pressure sensor 106 ′ and the intake temperature sensor 107 can be directly connected to the electronic control unit 105 ′. as a result , the labor of connecting lead wires can be eliminated . the operation of the first embodiment will be described below . the water pump 46 is mounted on an end of the crankcase 21 along the axis of the crankshaft 11 . the coolant supply passages 62 l , 62 r guide the coolant from the water pump 46 , and the coolant return passages 63 l , 63 r guide the coolant that is delivered out of the cylinder water jackets 60 l , 60 r of the cylinder barrels 16 l , 16 r . the supply passages 62 l , 62 r and the return passages 63 l , 63 r are formed parallel to the axis of the crankshaft 11 on both sides of the axes of the cylinder bores 18 l . . . , 18 r . . . . the cylinder water jackets 60 l , 60 r and the head water jackets 61 l , 61 r are formed such that the coolant supplied from the supply passages 62 l , 62 r returns from the cylinder water jackets 60 l , 60 r via the head water jackets 61 l , 61 r to the cylinder water jackets 60 l , 60 r . therefore , no piping is required outside of the engine for guiding the coolant from the water pump 46 to the cylinder barrels 16 l , 16 r , and no piping is required outside of the engine for delivering out the coolant from the cylinder heads 17 l , 17 r . therefore , the coolant piping around the engine is simplified . the cylinder water jackets 60 l , 60 r are divided into supply jacket portions 64 . . . and return jacket portions 65 . . . and are defined in the cylinder barrels 16 l , 16 r . the supply jacket portions 64 . . . communicate with the coolant supply passages 62 l , 62 r and also with the head water jackets 61 l , 61 r . the return jacket portions 65 . . . communicate with the coolant return passages 63 l , 63 r and also with the head water jackets 61 l , 61 r at positions spaced from the supply jacket portions 64 . . . . consequently , the coolant supply passages 62 l , 62 r and the coolant return passages 63 l , 63 r can be cast or drilled in one direction along the axis of the crankshaft 11 . therefore , the machinability for forming the passages is increased . the supply jacket portions 64 and the return jacket portions 65 . . . , with superposed portions surrounding substantial half of the cylinder bores 18 l . . . , 18 r . . . , are formed in the cylinder barrels 16 l , 16 r , which have the cylinder bores 18 l . . . , 18 r . . . arrayed in the axial direction of the crankshaft 11 . a plurality of supply and return branch passages 67 l , 67 r . . . ; 68 l , 68 r . . . are disposed between the crankcase 21 and the cylinder barrels 16 l , 16 r . the plurality of supply and return branch passages 67 l , 67 r . . . ; 68 l , 68 r . . . connect the portions of the supply jacket portions 64 . . . and the return jacket portions 65 . . . , which correspond to the cylinder bores 18 l . . . , 18 r . . . , to the coolant supply passages 62 l , 62 r and the coolant return passages 63 l , 63 r . thus , a passage structure for uniformly cooling portions corresponding to the respective cylinder bores 18 l . . . , 18 r . . . in a multicylinder engine can easily be constructed . by changing stepwise the diameters of the coolant supply passages 62 l , 62 r and the coolant return passages 63 l , 63 r , which extend linearly , the amount of the coolant flowing through the cylinder water jackets 60 l , 60 r and the head water jackets 61 l , 61 r , which correspond to the cylinder bores 18 l . . . , 18 r . . . , can be made more uniform . since the cylinder barrels 16 l , 16 r and the cylinder heads 17 l , 17 r are integrally formed by mutually joining the cylinder water jackets . 60 l , 60 r and the head water jackets 61 l , 61 r , a sand core in the shape of a succession of the cylinder and head water jackets 60 l , 60 r ; 61 l , 61 r is integrally formed for increased productivity for the cylinder barrels 16 l , 16 r and the cylinder heads 17 l , 17 r . the rod members 69 . . . divide the ring - shaped cavities , which is defined in the cylinder barrels 16 l , 16 r in surrounding relation to the cylinder bores 18 l . . . , 18 r . . . in a casting process , halfway into the supply and return jacket portions 64 . . . , 65 . . . . the rod members 69 are fitted into the cylinder barrels 16 l , 16 r from the cylinder heads 17 l , 17 r . therefore , the portion of the sand core , which corresponds to the cylinder bores 18 l . . . , 18 r . . . of the cylinder water jackets 60 l , 60 r , may be ring - shaped for easy sand removal to increase productivity and castability . in addition , the cylinder water jackets 60 l , 60 r can easily be divided into the supply jacket portions 64 . . . and the return jacket portions 65 . . . . the cylinder blocks 13 l , 13 r , which have the cylinder barrels 16 l , 16 r and the cylinder heads 17 l , 17 r integrally formed to provide communication between the cylinder water jackets 60 l , 60 r and the head water jackets 61 l , 61 r have a plurality of mounting bosses 34 . . . , the bosses 34 . . . extend from the surfaces of the cylinder barrels 16 l , 16 r , which are attached to the crankcase 21 , to the cylinder heads 17 l , 17 r . the bosses 34 . . . surround the cylinder bores 18 l . . . , 18 r . . . in the cylinder heads 17 l , 17 r . the cylinder blocks 13 l , 13 r are fastened to the crankcase 21 by the through bolts 30 . . . and the stud bolts 30 . . . extending through the mounting bosses 34 . . . . therefore , the cylinder barrels 16 l , 16 r and the cylinder heads 17 l , 17 r can simply be joined to the crankcase 21 . a gasket is not required between the cylinder barrels 16 l , 16 r and the cylinder heads 17 l , 17 r , bolts , which would otherwise be needed to keep the sealing ability of gaskets . as a result , the number of parts used is reduced , and the weight of the engine can be reduced . furthermore , the surrounding areas of the cylinder bores 18 l . . . , 18 r . . . can sufficiently be stiffened by the mounting bosses 34 . . . surrounding the cylinder bores 18 l . . . , 18 r . . . . the surrounding areas can withstand the tightening loads on the through bolts 30 . . . and the stud bolts 32 . . . that are inserted through the mounting bosses 34 . . . . the joint walls 35 . . . are integrally mounted on the cylinder blocks 13 l , 13 r . the joint walls 35 . . . join at least one of the two adjacent sets of the mounting bosses 34 , 34 , which are disposed on corresponding portions between the mutually adjacent cylinder bores 18 l , 18 l ; 18 r , 18 r arrayed in the axial direction of the crankshaft 11 . the joint walls 35 . . . join the pair of mutually adjacent mounting bosses 34 , 34 on the upper walls of the cylinder blocks 13 l , 13 r in the first embodiment . therefore , no bolts need to be disposed between the cylinder bores 18 l . . . , 18 r . . . , and the mechanical strength of the cylinder barrels 16 l , 16 r between the cylinder bores 18 l . . . , 18 r . . . can be increased by the joint walls 35 . . . . the distance between the cylinder bores 18 l . . . , 18 r . . . in the direction along the axis of the crankshaft 11 can be shortened for making the engine smaller in size . the rod guide tubes 100 l , 100 r , 101 l . . . , 101 r . . . axially movably insert the push rods and the pull rods of the valve operating device for transmitting the power of the crankshaft 11 . the rod guide tubes 100 l , 100 r , 101 l . . . , 101 r . . . and the joint walls 35 are disposed on the upper and lower sides of the cylinder blocks 13 l , 13 r . therefore , the rigidity of the cylinder blocks 13 l , 13 r is of an improved balance , preventing the inside diameter of the cylinder bores 18 l . . . , 18 r . . . in the cylinder blocks 13 l , 13 r from varying . fig1 shows a second embodiment of the present invention . the parts of the second embodiment corresponding to those of the first embodiment are denoted by identical reference characters . a pair of water pumps 46 rotatable by the crankshaft 11 is mounted on the crankcase 21 on one end of the crankshaft 11 , i . e ., a rear end of the crankshaft 11 in the second embodiment . the crankcase 21 has a coolant supply passage 62 l , a coolant supply passage 62 r , a coolant return passage 63 l and a coolant return passage 63 r . the coolant supply passage 62 l guides the coolant from one of the water pumps 46 into the supply jacket portion 64 in the cylinder water jacket 60 l in the left cylinder barrel 16 l . the coolant supply passage 62 r guides the coolant from the other of the water pumps 46 into the supply jacket portion 64 in the cylinder water jacket 60 r in the right cylinder barrel 16 r . the coolant return passage 63 l guides the coolant that is delivered out of the return jacket portion 65 in the cylinder water jacket 60 l in the left cylinder barrel 16 l . the coolant return passage 63 r guides the coolant that is delivered out of the return jacket portion 65 in the cylinder water jacket 60 r in the right cylinder barrel 16 r . these passages are defined parallel to the axis of the crankshaft 11 on both sides of the cylinder bores 18 l , 18 r as viewed in a figure projecting onto a plane perpendicular to the axes of the cylinder bores 18 l , 18 r . individual return conduits 110 l , 110 r are individually connected to the coolant return passages 63 l , 63 r . the return conduits 110 l , 110 r are connected in common to the first return conduit 72 . the coolant returning from the non - illustrated radiator is guided into the second return conduit 74 , which is connected in common to the water pumps 46 . a thermostat 73 is disposed between the first and second return conduits 72 , 74 . the thermostat 73 guides the coolant from the first return conduit 72 to the second return conduit 74 by bypassing the radiator when the temperature of the coolant is low . a steam conduit 76 for guiding steam evaporated by heating into an expansion tank 77 is connected to an upper portion of the cylinder block 13 l in communication with the coolant return passage 63 l . a third return pipe 76 for guiding the coolant that is condensed in the expansion tank 77 is connected to the second return conduit 74 . an oil cooler disposed in an oil filter 81 is supplied with the coolant from the water pumps 46 , . 46 via the supply conduit 82 , and the coolant from the oil cooler is returned to the thermostat 73 via a return conduit 111 . the second embodiment offers the same advantages as those of the first embodiment . while the embodiments of the present invention have been described above , the present invention is not limited to the above embodiments , but various design changes may be made without departing from the invention as defined in the scope of claims for patent . for example , the present invention has been described as being applied to a horizontally opposed multicylinder engine in the first and second embodiments described above . however , the present invention is also applicable to a v - shaped multicylinder engine or a single - cylinder engine . 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 .