Flywheels – When They Should Be Changed
Since your old clutch disc has had a direct relationship with the engine’s flywheel over many kilometres, inspecting the flywheel when things are apart is essential. The most important thing to check is the flatness of the flywheel because any small degree of warpage (or “runout”) will cause problems. Using a dial indicator or a straight edge across the flywheel at different points will give an indication if there’s any wobble.

Some manufacturers say runout up to .005 inch for every inch of flywheel diameter is acceptable, but many automotive engineers feel anything over .002 inches of runout per inch will create noticeable vibration, clutch chatter, and related issues. It’s a safe bet that any runout over .005 inches creates the risk of clutch failure from extreme vibrations, uneven clamping, etc.

Inspect the flywheel for cracks – especially around holes for mounting bolts. If cracks are deeper than surface scratches, the flywheel should be replaced in order to avoid the likely possibility it will shatter at high rpms and damage the transmission itself. Check for obvious “hot spots”, places where the flywheel may have overheated, or where clutch disc material was deposited on the flywheel surface. Check the starter motor teeth around the circumference of your old flywheel to make sure none have broken off. (These teeth are also known as the “ring gear”. On some flywheels, this part is replaceable separately.)

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