Mechanic Glossary

Feeler Gauge

A hand tool consisting of a set of thin steel blades of precise thicknesses used to measure narrow gaps between mechanical parts.

A feeler gauge is a simple tool containing a collection of thin metal blades. Each blade is ground to an exact thickness, usually marked in both thousandths of an inch and millimeters. Mechanics use them to check clearances between parts that must have a small gap to operate correctly. Common tasks include setting spark plug gaps, checking valve clearances on overhead cam engines, and measuring piston ring end gaps.

To use the tool, you slide different blades into the gap until you find one that fits snugly. You want to feel a slight drag as you pull the blade through the opening. If a blade slides in without any resistance, the gap is wider than the blade thickness. If the blade will not enter the gap, the gap is narrower. Always keep the blades clean and coated with a thin layer of oil to prevent rust, which changes the blade thickness.

Frequently Asked Questions

You should feel a slight resistance, often described as a "smooth drag," similar to pulling a sheet of paper out from under a heavy book. If you have to force the blade, it is too thick.
Yes. You can combine two or three blades to get the thickness you need. Just add the measurements printed on each blade together. Make sure the blades are completely clean, as dirt or oil between them will make the measurement inaccurate.
Flat feeler gauges use flat blades and are best for flat surfaces like valve lifters. Wire feeler gauges use round wire loops of different diameters and are designed for measuring spark plug gaps, where flat blades cannot sit flush against curved electrode surfaces.
If the gap is too small, the valves will not close completely when the engine gets hot, leading to lost compression and burnt valves. If the gap is too large, the valves will open late and close early, causing a loud clicking noise and reduced engine performance.
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