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How to Indicate a Bore True
Guide · Updated October 27, 2026
Indicating a bore puts the spindle on the bore's center, for boring, reaming or using it as a datum. This guide covers centering. For measuring bore size, see using a dial bore gage.
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Step by step
- Mount the indicator: Mount a test indicator offset in the spindle, or use a coaxial indicator.
- Sweep: Rotate the spindle by hand and note the readings at 0°, 90°, 180° and 270°.
- Split X: Move X by half the difference between the 0° and 180° readings.
- Split Y: Do the same for 90° and 270°.
- Re-sweep: Repeat until the total reading is within tolerance, then zero the DRO.
On a lathe
- In a 4-jaw chuck, indicate the bore and adjust the opposite jaws by half the reading.
- Tighten gradually and re-check.
Related: runout and TIR, cosine error.
Worked example: centering a 2.000 in bore on a mill
Zero the test indicator at the 9 o'clock position and rotate the spindle by hand. It reads +0.004 in at 3 o'clock, so the bore center is 0.002 in toward 3 o'clock (half the total indicator reading). Move the table 0.002 in in X, re-zero, and repeat for Y between 12 and 6 o'clock. Finish with a full rotation: under 0.0002 in total reading puts the spindle within 0.0001 in of the bore center.
FAQ
How do I center a mill on a bore?
Sweep a test indicator, move half the difference on each axis, and repeat.
Why move half the reading?
The indicator reads the change in diameter. Moving half of it centers the axis.
Is a coaxial indicator better?
It's faster. The dial stays still while the spindle turns.
How do I verify the indicator?
On gage blocks. 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.