Balance shaft

In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets the vibrations of unstable engines. Balance shafts are usually mounted low in the cylinder block and rotate at twice the speed of the engine. Balance shafts are most common in straight-4 engines, since these have fewer cylinders to balance the force of reciprocating masses. Large (greater than 2.0 L) straight-4s in particular are prone to rocking due to long-stroke rods or large-bore pistons.

General Motors' Quad 4 was notorious for vibration until balance shafts were installed, as was Chrysler's 2.4 L Neon engine.