Thread Gauges

Thread gauges are the go/no-go check that confirms whether a threaded feature — a tapped hole or a turned external thread — is within tolerance before it goes to assembly. The go gauge enters cleanly; the no-go gauge stops at the first turn. If both conditions are met, the thread passes. No measuring, no calculation, no ambiguity.

We stock a large range covering metric and imperial threads, including plug gauges for internal threads (6H tolerance) and ring gauges for external threads (6g tolerance). Common sizes run from M4 × 0.7 through M8 × 1.25 and well beyond, with right-hand and left-hand forms across the range. All gauges are manufactured to BS/ISO standards for use in production inspection, incoming QC, and machine setup.

With thousands of gauges held in UK stock, you can spec a single gauge for a repair job or kit out an inspection station for a full production run. Orders before 4pm ship same day.

Plug gauges vs. ring gauges — which do you need?

Use a plug gauge to check a tapped hole — it tests the internal thread. Use a ring gauge to check a stud, bolt or turned diameter — it tests the external thread. For a complete go/no-go check on a thread pair, you need both. If you're unsure which gauge matches your specification, each product page lists the thread form, tolerance class, and pitch for that gauge.

Plug gauges by thread series

A plug gauge is a fixed cylindrical gauge cut to the pitch and tolerance you're checking — 6H for a standard fit. The GO end has to wind fully into the tapped hole by hand; the NOGO end shouldn't enter more than a couple of turns. JBO manufactures the range we stock, ground to BS 919 / ISO 1502.

Browse plug gauges by thread series: metric coarse, metric fine, UNC/UNF (Unified), BSP (G), BSW, and NPT pipe threads.

Ring gauges by thread series

A ring gauge does the same job on the outside of a part — a stud, bolt, or turned diameter — at 6g tolerance. The GO ring should thread on by hand across its full length; the NOGO ring should stop within a turn or two. GO and NOGO rings are stocked separately in each series so you can order just the one you need.

Browse ring gauges by thread series: metric coarse GO / NoGo, metric fine GO / NoGo, Unified GO / NoGo, BSP GO / NoGo, and NPT.