Abstract: In an internal combustion engine in which air-intake valves reciprocate in parallel to the direction of reciprocal motion of pistons, an air-intake structure of an internal combustion engine includes an outward projection of the air-intake port that is reduced to achieve a downsizing of a cylinder head. An air-intake structure includes air-intake valves that are supported by a cylinder head having integrally formed air-intake ports so as to reciprocate in parallel with the direction of reciprocal motion of the pistons. Fuel injection devices are formed integrally with a throttle body on intake manifolds to be connected to the air-intake ports. The air-intake ports extend toward the cylinder head cover along a recess formed on the outer surface of the cylinder head that is connected to the intake manifolds at positions just before reaching the identical plane to a mating surface between the cylinder head and the cylinder head cover.

Abstract: An oil filter mounting structure in an internal combustion engine in which the outside a crankcase in the direction of a crankshaft is covered by a case cover. A spacer is interposed between the crankcase and the case cover. The spacer is formed with a lubrication system. An oil filter is mounted to an oil filter mounting surface of the spacer which is flush with a mating surface with respect to the crankcase. A part of the crankcase to which the oil filter mounting surface of the spacer faces is formed with a recess opening on top to which the oil filter is mounted. The resulting configuration provides an oil filter mounting structure in which an oil filter is reliably protected from stones. Further, oil channels are shortened to achieve a compact light-weight internal combustion engine.

Abstract: In an internal combustion engine in which air-intake valves reciprocate in parallel to the direction of reciprocal motion of pistons, an air-intake structure of an internal combustion engine includes an outward projection of the air-intake port that is reduced to achieve a downsizing of a cylinder head. An air-intake structure includes air-intake valves that are supported by a cylinder head having integrally formed air-intake ports so as to reciprocate in parallel with the direction of reciprocal motion of the pistons. Fuel injection devices are formed integrally with a throttle body on intake manifolds to be connected to the air-intake ports. The air-intake ports extend toward the cylinder head cover along a recess formed on the outer surface of the cylinder head that is connected to the intake manifolds at positions just before reaching the identical plane to a mating surface between the cylinder head and the cylinder head cover.

Abstract: An oil filter mounting structure in an internal combustion engine in which the outside a crankcase in the direction of a crankshaft is covered by a case cover. A spacer is interposed between the crankcase and the case cover. The spacer is formed with a lubrication system. An oil filter is mounted to an oil filter mounting surface of the spacer which is flush with a mating surface with respect to the crankcase. A part of the crankcase to which the oil filter mounting surface of the spacer faces is formed with a recess opening on top to which the oil filter is mounted. The resulting configuration provides an oil filter mounting structure in which an oil filter is reliably protected from stones. Further, oil channels are shortened to achieve a compact light-weight internal combustion engine.

Abstract: Connecting ports are respectively connected through joint plugs to inlet ports formed on the upper surface of an oil pump. The joint plugs have thin-walled cylindrical portions and circular flange portions formed at axially central positions of the respective cylindrical portions. The inlet ports are tapered in such a manner that the inner diameters of the inlet ports gradually increase toward opening ends thereof. One end portions of the joint plugs are preliminarily press-fitted with the inlet ports until the flange portions abut against the opening ends of the inlet ports, thereby achieving oil tightness. The other end portions of the joint plugs are oil-tightly connected through O-rings to the connecting ports.