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How to Use Small Hole Gages

Guide · Updated October 27, 2026

Small hole gages (split-ball gages) measure holes and slots too small for telescoping gages, typically about 0.125 to 0.500 in.

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Step by step

Select the gage: Choose the gage that covers the size.
Expand: Turn the knob until the ball halves touch the hole walls with a light drag.
Feel it: Move it in and out through the hole. The drag should be light and even.
Mic it: Measure across the ball with very light mic pressure. It compresses easily.
Repeat: Repeat and average the readings.

Worked example

You need to check a 0.2500 +0.0010/−0.0000 in reamed hole. Pick the gage whose range covers 0.250 in, open it until the ball halves just drag as you rock it across the hole, then lock it. Withdraw it straight out. Measure across the ball with a 0–1 in micrometer using the lightest possible feel (no ratchet): three readings of 0.2506, 0.2504 and 0.2505 in average 0.2505 in, inside the 0.2500–0.2510 in limits. Prove the method first by gaging a 0.2500 in ring gage or a slot built from a 0.2500 in gage block between two blocks; if that reads 0.2502 in, subtract 0.0002 in from your part readings, giving 0.2503 in.

Common problems

Readings that wander by more than 0.0005 in usually mean too much mic pressure squeezing the halves, a gage set too tight so it does not pass the true diameter, or a gage that never crossed the center line, so it measured a chord. Bellmouthed or tapered holes need readings at the top, middle and bottom.

Tips

Measure in two directions to see out-of-round.
For tighter work, check holes with pin gages instead.
Check your mic feel against gage blocks.

Related: pin gages, telescoping gages.

Worked example: measuring a 0.187 in hole

Expand the small hole gage in a 3/16 in hole until there is a light drag, then measure across the balls with a 0–1 in micrometer. If the micrometer reads 0.1875 in, the hole is 0.1875 in. Repeat twice and average; small hole gages are typically good to about 0.0002–0.0005 in in practiced hands.

Common problems: overtightening wears the gage and reads small; tilting reads large.

Second worked example: checking a 3/16 in hole for size and ovality

Set the small-hole gage in a nominal 0.1875 in hole, lock it with light drag, and mike the ball halves. Readings of 0.1882 in at 0° and 0.1879 in at 90° mean the hole is 0.0004–0.0007 in over nominal and 0.0003 in oval. Repeat each reading three times; a spread over about 0.0002 in usually means inconsistent feel rather than a real size change.

Mike across the ball, not the flat of a split-ball gage, and keep micrometer pressure light, because the gage halves can spring. Confirm your feel on a ring gage or a gage-block-and-jaw setup of the same size before trusting tenths.

Quick shop check

Common problems: inconsistent readings mean you are not feeling the same drag. Practice on a ring gage to calibrate your feel.

FAQ

What size do small hole gages cover?

Usually about 0.125 to 0.500 in in a set.

Why are small hole readings low?

Too much mic pressure squeezes the ball halves.

Can they measure slots?

Yes. Flat-bottom types are better in slots.

How do I verify one?

Measure a pin gage or a gage block slot of known size. Build any reference size free with the Gage Block Calculator at gageblockcalc.com.

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