How to Choose the Right Thread Gauge

Specifying a thread gauge comes down to three decisions: the standard, the class or tolerance, and whether you need a plug or a ring.

1. Identify the standard

Check the drawing or component spec for the governing standard, for example DIN ISO 1502, ANSI/ASME B1.2, ANSI/ASME B1.20.1, ISO 228-2 or ASME B1.5. This fixes the thread angle, form and how the gauge is made.

2. Identify the class or tolerance

Most standards define a tolerance class, such as 6H/6g for metric threads, class 2A for unified threads, or 2G/3G/4G for ACME threads. A tighter class means a smaller allowable range, so it needs to be confirmed before the gauge is made.

3. Plug or ring

A thread plug gauge checks an internal thread, such as a nut or tapped hole. A thread ring gauge checks an external thread, such as a bolt or stud. Most jobs need both a GO and a NOT-GO member of whichever type matches the feature being checked.

Still not sure?

Share the thread size, standard, or even just a drawing on our quote page, and we will confirm the right class and gauge type for your part.