Feeler Gauge

It measures valve clearances, spark plug gaps and other tolerances. Blades are marked in millimetres or inches. Wire gauges are better for worn spark plug electrodes. To check valve clearance, the engine is turned so the cam lobe points away from the follower, and the blade is slid between cam and follower or rocker and valve stem. Some engines with shim adjustment need the measured value to calculate a new shim size. Feeler gauges are also used to set gaps on older ignition points and to check brake and clutch play.

The correct blade should slide through the gap with light resistance.

Bent or rusty blades give wrong readings. Clean blades before use.