Abstract: A bolt fastening structure for an internal combustion engine includes bolt fitting holes formed in a cylinder of the engine. An inner wall of a top portion of a crankcase includes boss portions having openings, as an extension to the corresponding bolt fitting holes, with internal threads formed therein. The crankcase further includes hollow recesses formed thereon between the boss portions and crankshaft rotational support portions. Fastening bolts, each having an external thread formed at a tip portion thereof, are extended through the bolt fitting holes, and engaged with the internal threads of the boss portion. The hollow recesses include portions of the openings of the boss portions having the internal threads opened therein.

Abstract: A bolt fastening structure for an internal combustion engine includes bolt fitting holes formed in a cylinder of the engine. An inner wall of a top portion of a crankcase includes boss portions having openings, as an extension to the corresponding bolt fitting holes, with internal threads formed therein. The crankcase further includes hollow recesses formed thereon between the boss portions and crankshaft rotational support portions. Fastening bolts, each having an external thread formed at a tip portion thereof, are extended through the bolt fitting holes, and engaged with the internal threads of the boss portion. The hollow recesses include portions of the openings of the boss portions having the internal threads opened therein.

Abstract: An arrangement structure of a starter motor in an all-terrain vehicle in which a minimum road clearance is secured sufficiently and lowering of the center of gravity and adequate weight distribution of the vehicle can be achieved. The starter motor is arranged in an area at the front portion of the vehicle with respect to a vertical line including a center point of a crankshaft of an engine, downwardly of a horizontal line including the center point of the crankshaft of the engine, and upwardly of a horizontal line including a lowermost portion of the crankcase of the engine in the vehicle.

Abstract: An arrangement structure of a starter motor in an all-terrain vehicle in which a minimum road clearance is secured sufficiently and lowering of the center of gravity and adequate weight distribution of the vehicle can be achieved. The starter motor is arranged in an area at the front portion of the vehicle with respect to a vertical line including a center point of a crankshaft of an engine, downwardly of a horizontal line including the center point of the crankshaft of the engine, and upwardly of a horizontal line including a lowermost portion of the crankcase of the engine in the vehicle.

Abstract: An intake port and an exhaust port are formed without changing a passage sectional area or its shape a little and without increasing an entire height of an engine. A seat machining part of a valve guide fixing seat of the intake port and the exhaust port are arranged at the cylinder head and are bent in any one of the right side direction or the left side direction as seen from the center of the valve. With such an arrangement, a minimum thickness for the seat machining can be assured, the intake port and the exhaust port do not change the passage sectional area or the sectional shape a little and so it is not necessary to increase the entire height of the engine.

Abstract: An arrangement for improving the cooling performance of an engine by suitably arranging an exhaust pipe and a valve mechanism with respect to the inclined direction of a cylinder block. A cylinder block is inclined in the rightward or leftward direction of a vehicle body, and an exhaust pipe and a valve mechanism are disposed along the inclined side wall of the cylinder block. The opposed side wall of the cylinder block, which has a large space in the vertical direction and exerts a large effect on the cooling performance of the engine, is less susceptible to thermal effects from the exhaust pipe and the valve mechanism, to thereby improve the cooling performance of the entire engine.

Abstract: A shaft having a sprocket fixed at one end thereof is reliably assembled to the supporting wall of an engine, with a reduced number of parts and man-hours required for assembly. A camshaft having a sprocket fixed at one end thereof is installed by engaging a pair of ball bearings with supporting holes on supporting walls of the engine. A crescent-shaped step partially cut in the loosing direction of an endless chain is formed on one supporting hole. An outer race of one ball bearing on the camshaft is engaged with the step. An endless chain is wound around the sprocket, and the one ball bearing is moved into the supporting hole, and the other ball bearing is correctly engaged with the other supporting hole, thereby completing assembly of the camshaft.

Abstract: A power unit for a vehicle constructed by combining an internal combustion engine and a transmission integrally is provided. A crankshaft rotatably supported in front and rear crankcases of the internal combustion engine is positioned biased slightly toward right side from the center of the crankcases, a cylinder block and a cylinder head is inclined toward the left so as to extend obliquely leftward and upward from the crankshaft, on the left side of the crankshaft are arranged a main shaft and an auxiliary shaft of the transmission in turn, at a middle and upper portion between the main and auxiliary shafts a reverse shaft, and below the auxiliary shaft a driving shaft. On the right side of the crankshaft is arranged a cam shaft. These shafts are parallel with each other and directed in the vehicle running direction.

Abstract: A chain guide structure having, in combination, a chain cover and a chain guide member which can easily and securely be placed on the chain cover for temporary assembling, and capable of improving the efficiency of the assembling work. A chain guide structure for guiding a chain wound round a driving sprocket supported on the sidewall of a crankcase and covered with a cover attached to the sidewall includes an elastic chain guide member having a substantially semicircular shape opening toward the rear and supported on the inner surface of the cover so as to extend along the chain. At least two ribs are formed on the inner surface of the cover so as to hold the chain guide member by elastically deforming the chain guide member so that the width of the open end of the chain guide member is changed.