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How to Set a Bore Gage

Guide · Updated October 28, 2026

A dial bore gage is a comparator: it reads how far a bore is from the size you set it to. Setting it accurately matters more than how you swing it, so pick the best master you have: a ring gage, a gage block stack between jaws, or a micrometer locked at size.

Build the setting stack

Build the exact bore size from your gage blocks, then set the bore gage between block jaws. The free Gage Block Calculator builds the stack.

Step by step

Fit anvils: Choose the extension and spacers so the gage covers the nominal size with some preload.
Choose a master: Ring gage is best; gage blocks with caliper jaws in a holder next; a mic locked at size is quickest.
Insert in the master: Tilt the gage in, then rock it through the true diameter.
Find the turnaround: The needle stops and reverses at the smallest reading: that is the true size.
Zero the bezel: Rotate the bezel to zero at the turnaround; note the revolution counter.
Repeat: Check two or three times; it should return within a division.
Measure: In the part, the turnaround reading shows the difference from the setting size.

Worked example

The bore is 2.5000 +0.0008/−0.0000 in. Build 2.5000 in from blocks (for example 2.000 + 0.500) and clamp it between half-round jaws in a holder. Set the bore gage and zero it at the turnaround. In the part, the turnaround reads 0.0003 in past zero on the large side, so the bore is 2.5003 in, inside tolerance. Remember the plunger direction: on most bore gages, a needle moving counterclockwise means larger. Confirm the direction by checking a slightly larger master once.

Temperature

Gage, master and part should be at the same temperature. A steel bore at 78 °F with a 68 °F master reads about 2.500 × 0.0000064 × 10 = 0.00016 in large.

Common problems

Drift means the gage warmed in your hand; hold it by the insulated handle. A jumpy reading means a burr or chip on the anvil. Wrong sign means you misread the plunger direction.

Tips

Using it: use a dial bore gage.
Checking it: check a bore gage with gage blocks.
Rings: ring gage basics.

FAQ

What is the best way to set a bore gage?

On a ring gage of the nominal size.

Can I set it with a micrometer?

Yes, but accuracy depends on the mic; check the mic on gage blocks.

What is the turnaround?

The reversal point when rocking, which is the true diameter.

Which needle direction is larger?

It varies by gage; verify with a larger master.

How often should I re-zero?

At the start of a job and after any knock or temperature change.

Related guides and calculators

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