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How to Use a Dial Bore Gage
Guide · Updated October 27, 2026
A dial bore gage is a comparator: it reads how far a bore is from the size you zeroed it to. Good results come down to a careful zero and the rocking technique. To verify the gage itself, see checking a bore gage with gage blocks.
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Step by step
- Fit the right anvil: Choose the anvil and washers so the nominal size sits mid-range, with about half a turn of preload.
- Set the master size: Use a setting ring, or a micrometer locked at nominal. The mic should have been checked with gage blocks.
- Zero at the minimum: Rock the gage through the setting master, find where the needle reverses (the minimum), and set the bezel to zero there.
- Measure the bore: Insert the gage, rock it the same way and read at the reversal point.
- Read the sign: Read clockwise or counterclockwise of zero against the dial's marking to get the bore over or under nominal.
- Map the bore: Measure at top, middle and bottom, and at 0° and 90°, to see taper and out-of-round.
Rocking tips
- Rock slowly along the bore axis. The minimum is the true diameter.
- Center it side to side as well. The centralizer does most of this.
- Hold it by the insulated handle so body heat doesn't creep into the reading.
Common errors
- Zeroing at the peak instead of the minimum
- Too little preload, so the reading falls off the dial
- Measuring hot parts right off the machine
Related: checking a bore gage, gage block accessories, temperature compensation.
Worked example: measuring a 1.500 in bore
You need to check a bore that should be 1.500 in. Fit the anvil and extension that cover 1.500 in, then set the gage to zero in a 1.500 in setting ring, or in a micrometer locked at 1.500 in. Rock the gage in the ring and zero the dial at the turnaround point (the smallest reading). Move it to the part, put it in to the measuring depth, and rock it slowly through the axis. Read the turnaround point again. If the needle reverses 0.0008 in on the plus side of zero, the bore is 1.5008 in at that depth. Check near the top, middle, and bottom, and turn the gage 90° each time to catch taper and out-of-round. Check zero in the ring again at the end.
FAQ
Why do you rock a bore gage?
The gage reads smallest when it's square to the bore, so the minimum while rocking is the true diameter.
How do you zero a dial bore gage?
In a setting ring, or between micrometer anvils set to nominal. Rock it there and zero at the reversal point.
Can I set a bore gage with gage blocks?
Yes. A gage block stack between block-set jaws in a holder makes an accurate setting size. Build the stack with the free calculator at gageblockcalc.com.
How accurate is a dial bore gage?
Typically 0.0001 in to 0.0005 in resolution, depending on the dial. The setting master usually limits accuracy.
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General shop information only — follow your gage manufacturer's instructions and your quality system.