Balance Shaft

Four-cylinder engines above about two litres often use a pair of balance shafts turning at twice crankshaft speed. Three-cylinder engines often use one shaft at crankshaft speed. They are driven by the crankshaft through a chain, belt or gears, and sometimes form a module with the oil pump. Without them, larger four-cylinder engines would shake noticeably at higher revs.

Offset weights on the shaft create forces equal and opposite to the engine's unbalanced forces, so the vibrations cancel out.

A worn balance shaft drive or bearing can cause rattles, vibration and, on some engines, oil pressure problems. If the drive is wrongly timed after repairs, the engine vibrates more than before.