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How to use GO / NO-GO thread gauges: thread inspection and incoming acceptance essentials

2026-10-02

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why thread inspection needs gauges, not eyes or fingers

Thread quality cannot be judged by sight or feel: pitch-diameter oversize, wrong pitch, flank-angle error and burrs on the flanks are all invisible, and they surface only in assembly — a bolt that will not start, a joint that torques out but never clamps, or threads that strip after a few days of vibration. The reason is that a thread fit is set by five parameters at once: major, minor and pitch diameter, pitch and flank angle, with the pitch diameter as the core of the fit — it governs the clearance or interference between mating threads. Any parameter out of tolerance makes engagement gritty or loose, yet measuring a single parameter takes a toolmakers microscope or a thread micrometer, minutes per part — no incoming-inspection bench can sustain that on lots of thousands. GO / NO-GO thread gauges exist precisely for this: two limit gauges compress the question of whether all parameters jointly fall inside the tolerance class into a few seconds of screwing motion — no readings, no bench equipment, and any storekeeper or inspector can run them. That is why limit gauging, not instrument measurement, is the standard method for fastener incoming acceptance.

The accept rules: full engagement on GO, two turns maximum on NO-GO

The gauge logic is that a conforming thread must pass GO and refuse NO-GO. The GO gauge represents the maximum-material limit: a GO ring gauge on external threads or a GO plug gauge on internal threads must run smoothly over the full thread length to the end without binding — it checks the functional pitch diameter, the combined effect of pitch diameter, pitch and flank angle. The NO-GO gauge checks only the single pitch diameter: to GB/T 3934 it may enter no more than two thread pitches — about two turns — and the hand must feel no appreciable tightening torque; if NO-GO takes more than two turns, the pitch diameter is out of tolerance and the part is rejected. Match the gauge system to the standard: metric thread gauges to GB/T 3934 (aligned with ISO 1502), inch UNC/UNF ring and plug gauges to ASME B1.2; tolerance classes follow GB/T 197 — commercial fasteners pair a 6g external with a 6H internal thread. Watch plated parts: a 5-8 um electro-plated layer eats into tolerance, so reputable mills allow a pre-plate oversize class or fully re-gauge after plating; on incoming inspection, a gritty GO engagement on zinc-plated screws usually means the coating is too thick.

Sampling, gauge wear control and what to do with rejects

Run four steps: clean oil and burrs off the threads and eyeball the crests for damage; run the GO gauge over the full length — it must reach the end freely and back off without binding; the NO-GO gauge must not exceed two turns by hand with no felt torque; then record batch numbers and the verdict. Two prerequisites matter. First, gauges themselves need scheduled calibration — ring and plug gauges wear, and a worn GO gauge passes rejects — so send them for periodic calibration and label the validity date. Second, sample to GB/T 2828.1 general inspection level II; thread acceptance is a functional defect, so AQL 1.0-2.5 is typical, or per agreement, tightened on large lots. On verdicts: GO fails while NO-GO passes — usually pitch diameter oversize or flank-angle error; both gauges run loose — suspect mixed stock. For disputed lots, re-measure the single pitch diameter with a thread micrometer or send samples to a third-party lab, and check the EN 10204 3.1 certificate against the heat numbers so the claim file is complete in one pass. Need fastener supply with gauge-recheck reports, spec sheets or sample fittings? Contact JIALI Industrial (164.hk) with sizes and acceptance requirements for a matching quotation.

How to use GO / NO-GO thread gauges: thread inspection and incoming acceptance essentials

Why thread inspection needs gauges, not eyes or fingers

The accept rules: full engagement on GO, two turns maximum on NO-GO

Sampling, gauge wear control and what to do with rejects

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