Identifying a Thread: Which Bolt Is That, Actually?

Put a caliper across the widest part of the thread and read it. An M8 bolt measures very close to 8 mm; a 5/16 inch bolt measures about 7.9 mm. That 0.1 mm is the entire difference and a caliper alone will not settle it — which is exactly why the pitch matters more than the diameter for identification. Measure the diameter to narrow it to two or three candidates, then use the pitch to decide. Measure a clean unworn part of the thread, not a damaged end or the run-out near the head. And measure a BOLT rather than trying to measure the hole: an internal thread is far harder to measure directly, and the published bore-measurement blueprint exists for exactly that reason.

A thread pitch gauge is a fan of thin steel leaves, each cut to one pitch. Try them against the bolt until one seats with no light showing between the teeth anywhere along it. Counting threads per inch with a rule works but is slow and error-prone, and the commonest mistake is off-by-one at the ends. The gauge is unambiguous: the right leaf disappears into the thread and every wrong one rocks or shows light. Metric leaves are marked in millimetres of pitch (1.25, 1.5) and imperial ones in threads per inch (18, 20, 24). A set usually carries both, and which kind of leaf fits is itself half the identification.

Once a fastener is identified, it costs nothing to keep it that way. Sort by thread into labelled compartments and the whole problem disappears permanently. Keep imperial fasteners physically separate from metric ones rather than mixed and labelled. The near-miss pairs are what cause the damage, and they only cause it when both are within reach of the same hand. Where a machine uses an unusual thread, tape a labelled sample to it or write the thread on the maintenance note. Working it out from scratch takes ten minutes every time, and stripping the hole because you guessed takes an afternoon to repair.