Abstract: An internal combustion engine is equipped with a crankcase ventilation system in which fresh air flows through a crankcase. The cylinder block of the engine is formed with a fresh air suction port whose one end is opened to the chamber of a crankcase in order to establish communication between a rocker cover chamber and the crankcase chamber. The one end of the fresh air suction port is located in the vicinity of the peripheral surface of a counterweight of a crankshaft. The rotation of the counterweight develops vacuum thereby to suck fresh air from the rocker cover chamber through the fresh air suction port into the crankcase chamber, thereby effecting ventilation in the crankcase.

Abstract: A camshaft driving arrangement for a DOHC engine comprises two pairs of cam sprockets, each pair being mounted to each of the camshafts at one end thereof, and two idler gears, each being rotatably mounted to one of the cylinder heads. A first camshaft driving chain drivingly interconnects one of the idler gears and the adjacent pair of the cam sprockets. On the other hand, a second camshaft driving chain drivingly interconnects the other of the idler gears and the other pair of the cam sprockets. The second camshaft driving chains have different running planes which are axially spaced along the axis of the crankshaft.