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How to Use a Telescoping Gage
Guide · Updated October 27, 2026
Telescoping gages transfer a bore size to a micrometer. They have no scale of their own, so the result comes down to feel and technique.
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Step by step
- Pick the size: Choose the gage whose range covers the bore.
- Insert at a tilt: Compress the plungers, lock lightly, insert at a slight angle and release.
- Lock at the diameter: Rock the handle through center while tightening the lock lightly, so the plungers settle at the true diameter.
- Roll out: Roll it out through center in one smooth motion.
- Mic it: Measure across the plungers with light micrometer feel, then repeat 2–3 times.
Tips
- Light lock pressure works best. Over-tightening shifts the plungers.
- Check your feel on a ring gage or a gage block stack between jaws.
- For production work, use a dial bore gage.
Related: using a dial bore gage, small hole gages.
Worked example: measuring a 1.250 in bore
Lock the gage lightly in the bore, rock it through center and lock it. Measure across with a 1–2 in micrometer; a reading of 1.2505 in is the bore size. Take three readings at different depths to check taper.
Telescoping gages typically repeat within about 0.0005 in; for tighter work use a dial bore gage.
Fuller worked example: three readings in one bore
In a nominal 0.875 in bore, lock the telescoping gage and mike it three times: 0.8763, 0.8761 and 0.8764 in. The spread is 0.0003 in, so report about 0.8763 in and treat the last tenth as uncertain. Readings 90° apart of 0.8763 and 0.8758 in show about 0.0005 in of ovality.
Rock the gage through center with light, steady pressure, and mike it with the ratchet so the plunger does not compress. Confirm your technique in a ring gage before trusting tenths.
FAQ
How accurate are telescoping gages?
About ±0.0005 in with good technique, which depends heavily on feel.
Why do my readings vary?
Inconsistent lock pressure, a crooked roll-out or mic feel. Practice on a known ring.
Telescoping vs small hole gage?
Telescoping gages cover about 5/16 in and up. Small hole gages cover smaller bores.
How do I practice?
Measure a known size, such as a ring or a gage block stack between jaws. 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.