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Thread Pitch Gage Basics
Guide · Updated October 27, 2026
A thread pitch gage (screw pitch gauge) tells you the pitch of a thread: how many threads per inch, or millimetres per thread. It does not measure the thread size or pitch diameter. Here's how to get a confident ID.
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Step by step
- Clean the thread: Brush out chips and dirt so the leaf seats in the thread.
- Measure the major diameter: Mic or caliper the OD to narrow it to inch or metric sizes.
- Try leaves: Lay a leaf along the thread axis, parallel to it, with the teeth fully seated.
- Check against light: Hold it up to a light. With the correct leaf, no light shows along at least 1/2 in of engagement.
- Confirm the system: Close pairs like 1.25 mm and 20 TPI can look alike. Check over a longer length.
Close pitches that fool people
- 1.0 mm vs 24/25 TPI
- 1.25 mm vs 20 TPI
- 1.5 mm vs 16/17 TPI
- 1.75 mm vs 14/15 TPI
- Longer engagement shows the mismatch.
When a pitch gage isn't enough
- Pitch diameter: use three-wire measurement or thread mics.
- Fit and acceptance: use thread plug and ring gages, as described in go/no-go gaging.
Related: three-wire thread measurement, tapping speeds and feeds, go/no-go gaging.
Worked example: identifying an unknown bolt
A bolt comes in with no markings. Brush the threads clean and caliper the major diameter: it reads about 0.310 in, which could be 5/16 in or M8. Try the 18 TPI leaf first: it seats but light shows at the far end. Try the 1.25 mm leaf: it seats fully with no light along the whole engagement. So the bolt is metric, M8 x 1.25. Confirm by threading it into a known M8 nut by hand; it should run in freely without binding.
FAQ
How do you read a thread pitch gage?
Find the leaf that meshes with no light showing. The number stamped on it is the TPI (inch) or pitch in mm (metric).
Can a pitch gage measure thread size?
No. It gives pitch only. Measure the major diameter separately and match it to a thread chart.
How do I tell metric from inch?
Use the major diameter and check the pitch over a long length. Near-equal pitches drift out of mesh over several threads.
Does a pitch gage check thread quality?
No. Use three wires for pitch diameter, or go/no-go thread gages for acceptance.
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General shop information only — follow your gage manufacturer's instructions and your quality system.