Shim

A shim is a thin, precisely sized piece of metal used to set a clearance or spacing between parts. On a motorcycle the best-known shims are valve shims: in engines with bucket tappets, a small disc of known thickness sits on or under the bucket and sets the gap between the cam and the valve.

When valve clearance is out of spec, the old shim is measured with a micrometer and replaced by one that is thinner or thicker by the required amount. Shims come in steps of a few hundredths of a millimetre.

Shims are also used in wheel bearings, gearbox shafts and suspension dampers, where stacks of shims control damping. Always record what you remove so you can return to the original setup.