Crankshaft

The crankshaft is the main rotating shaft of the engine. The connecting rods attach to offset crankpins, so as the pistons are pushed down by combustion the crankshaft turns, converting linear motion into rotation. The power then flows through the primary drive, clutch and gearbox, and finally via chain, belt or shaft to the rear wheel.

Counterweights (flywheels) on the crank smooth out the power pulses and balance the moving parts. The angle between crankpins, known as the crank or firing order, gives each engine its character, such as the 270° crank in many modern parallel twins. The crank also drives the camshafts, alternator and often the oil pump.

Worn main bearings cause knocking and low oil pressure. In two-strokes, worn crank seals let air in and cause lean running. Crank work usually means splitting the crankcase.