How to use a feeler gauge to check gaps correctly
The feeler gauge is the simplest tool for measuring narrow gaps with precision. This guide shows how to check gaps and valve clearance correctly, choose the right blade set and recognise when a blade is worn out.
View feeler gaugesHow does a feeler gauge work?
A feeler gauge is a set of thin steel blades, each ground to a precise thickness and folded fan-like into a holder. Every blade is marked with its size, such as 0.10 or 0.50 mm. To measure, you slide the matching blade into the gap and judge the resistance.
You have found the correct size when the blade moves through the gap with a light, steady drag - the so-called snug fit. A blade that slips through with no resistance is too thin; one that jams is too thick.
- Metric sets: 0.05 mm to 1.00 mm in fine steps.
- Imperial sets: 0.0015" to 0.025" for inch-based specifications.
- Combination blades allow intermediate values by stacking two blades.
How do you check valve clearance and gaps?
The classic job is checking valve clearance on an engine. The piston must be at top dead centre of the compression stroke so the valve is fully closed. The blade is then slid between the cam heel and the valve follower.
When checking gaps on components or body panels, insert the blade at several points to judge how even the gap is. Always follow the manufacturer specification - for engines the values usually apply to a cold engine unless stated otherwise.
Which blade set fits and when is it worn out?
The right blade set depends on measuring range, blade count and blade length. Tight valve covers call for narrow, angled or bent blades, while open gap measurements suit longer straight blades. Stainless versions make sense where moisture or fuel contact is likely.
- Blade count: 13 blades for standard work, up to 20 for fine steps.
- Length: 100 mm standard, 150‑300 mm for deep or hard-to-reach gaps.
- Material: hardened spring steel or rust-free stainless steel.
- Special shape: angled blades for cramped engine bays.
Feeler gauges wear through abrasion, bending and corrosion. A blade with a thinned edge, a bend or a pitted surface reads too small and must be replaced. Check critical blades regularly against their nominal size with a micrometer.
Frequently asked questions
How do I know I have the right blade?
The correct blade slides through the gap with a light, steady drag - it feels snug but does not jam. If it drops through loosely it is too thin; if it locks up it is too thick.
Can I combine several blades?
Yes. Place two clean blades together and their thicknesses add up. That lets you reach in-between sizes that are not present as a single blade in the set.
Should I measure valve clearance hot or cold?
Valve clearance is normally checked on a cold engine unless the manufacturer states otherwise. Thermal expansion changes the clearance noticeably, so always follow the workshop specification.
When should I replace a feeler gauge?
When a blade is bent, rusted or thinned at the edge and deviates by more than ± 0.01 mm from its nominal size. Such blades read too small and compromise accuracy.
Looking for the right feeler gauge?
We supply metric and imperial feeler gauges in spring steel and stainless steel - in various lengths and blade counts for valve clearance and gap measurement.
Precisely made
Blades to DIN 2275, toleranced to ± 0.01 mm.
Tough material
Hardened spring steel and rust-free versions.
Complete sets
13 to 20 blades for fine graduations.
Expert advice
Our specialists help you make the right choice.