Dual-mass Flywheel

Dual-mass flywheels are common on diesels and powerful petrol engines with manual or dual-clutch gearboxes. They reduce gearbox rattle and vibration at low engine speed, but they are a wear part. The springs and grease inside cannot be serviced. Manufacturers give a maximum allowed free play and rock between the two masses.

The primary mass bolts to the crankshaft and the secondary mass carries the clutch. Curved springs and dampers between them let the two halves twist slightly against each other.

Signs of wear include rattling at idle, judder when pulling away and knocking when switching off. It is usually checked whenever the clutch is replaced. Long periods of low-speed driving in high gears and frequent stalling put extra load on a dual-mass flywheel.