Balance Shaft (Counterbalancer)

A balance shaft, also called a counterbalancer or counterweight shaft, is a shaft with an offset weight that spins in time with the crankshaft. It is positioned and timed so that its weight moves against the forces created by the pistons and connecting rods, cancelling part of the engine's vibration and making it feel smoother.

Balance shafts are common on single-cylinder engines and parallel twins, which have strong natural vibration. Some engines use one shaft, others two. Note that a balance shaft is not the same as the countershaft, which is the gearbox output shaft carrying the front sprocket.

Increased vibration after an engine rebuild can mean the balancer was timed incorrectly; always align the timing marks as shown in the service manual. On some older off-road singles the balancer chain needs periodic adjustment.