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How to Check Radius Gages

Guide · Updated October 27, 2026

Radius gages (fillet gages) are leaves cut to a known radius. Use them often enough and edges get bent and worn. Checking them takes nothing more than gage pins or a gage block stack of twice the radius and a good light.

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Need a 2 × R gap for a convex leaf? Get the stack from the free Gage Block Calculator. Free, no login.

Reference sizes

Radius	Reference diameter (2 × R)	Stack for jaw gap (81-piece)
1/16 in	0.125 in	Pin only (below set range)
1/8 in	0.250 in	0.250
3/16 in	0.375 in	0.250 + 0.125
1/4 in	0.500 in	0.500
5 mm	10 mm	10 (88-piece)

Step by step

Clean the leaf and reference: Wipe the leaf and the pin or ring.
Pick the reference: Use a gage pin of 2 × R for concave leaves, or a ring or jaw gap of 2 × R for convex leaves.
Seat the leaf square: Hold the leaf square to the pin axis, touching along the arc.
Check against light: Look for light along the arc. It should be even and thin, or not there at all.
Tag or replace: Mark bent or worn leaves and replace them.

Tips

Check the pins first with a micrometer that's been checked with gage blocks. See how to check a micrometer.
Jaws and holders for inside gaps are covered in gage block accessories.
Store leaves closed and lightly oiled. See storage.

Worked example: checking a 1/8 in leaf

You pull the 1/8 in (0.125 in) leaf from a set and want to confirm it. The internal (concave) end needs a 0.250 in round, so take a 0.250 in gage pin, wipe it and the leaf clean, and seat the pin in the arc. Hold both up to a bright light at eye level. Light leaking at one end of the arc but not the other usually means the leaf is bent; rock it slightly and recheck. An even hairline along the whole arc is acceptable for a comparison gage. If light shows at both ends with contact in the middle, compare against the 0.249 in and 0.251 in pins to see which way it's off. Mark or retire any leaf that fails, and don't straighten it by hand.

FAQ

How do you check a radius gage?

Test each leaf against a known round: a gage pin or a stack-built part of diameter 2 × R. Hold it to a light. An accurate leaf shows no light, or an even thin line, along the whole arc.

What pin checks a 1/8 inch radius leaf?

A 0.250 in gage pin, because the diameter is twice the radius. For a concave (internal) leaf, the pin sits inside the arc. For a convex leaf, use a ring or bore of that size, or the inside of a gage-block jaw setup.

How accurate are radius gages?

They're comparison gauges, good for spotting a few thousandths of mismatch by light gap. For tighter work, use an optical comparator or a CMM.

Can gage blocks check a radius gage?

Indirectly. Use gage blocks to verify the gage pins, or with accessory jaws to set an inside gap of 2 × R for convex leaves. Build stacks free at gageblockcalc.com.

Why does my radius gage show light at the ends only?

The leaf radius is slightly smaller than the pin, or the leaf is worn or bent. Try the next size and compare.

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General shop information only — follow your gage manufacturer's instructions and your quality system.