What Is a Thread Gauge? Types and Uses

A thread gauge is a tool used to check a screw thread. The term covers two different kinds of tool: fixed GO / NOT-GO limit gauges that decide whether a thread is within tolerance, and thread pitch gauges (leaf gauges) that only identify the pitch.

Thread plug gauges

A thread plug gauge checks an internal thread, such as a tapped hole or a nut, with a GO member that must screw in and a NOT-GO member that must not.

Thread ring gauges

A thread ring gauge checks an external thread, such as a bolt or a stud. The GO ring must screw on, and the NOT-GO ring must not go on more than two turns.

Taper thread gauges

Taper pipe threads such as NPT and BSPT seal by the taper, so they are checked with a basic or min/max step gauge instead of a GO / NOT-GO pair.

Thread pitch gauges

A thread pitch gauge is a set of thin blades that you match against a thread to find its pitch. It helps you identify an unknown thread but does not tell you whether the thread is in tolerance.

Plain gauges and snap gauges

Plain plug and ring gauges and snap gauges check plain, non-threaded diameters such as bores and shafts.

Which standard does the gauge follow?

Every thread gauge is made to a thread standard, such as DIN ISO 1502 for metric threads or ANSI/ASME B1.2 for unified threads, so the gauge has to match the standard on your drawing.