How a GO / NOT-GO Thread Gauge Works

A limit gauge, also called a go/no-go gauge or screw thread gauge (gage in US spelling), doesn't give you a measured number. It gives you a pass or fail decision at the edge of the acceptable size range, using two separate members: a GO member and a NOT-GO member.

The GO member

The GO member must enter and pass the complete length of the thread when screwed on by hand, without excessive force. If it does, the feature is within the accepted size.

The NOT-GO member

The NOT-GO member must not enter more than two turns from either end, screwed on by hand without excessive force. If it goes further than that, the part is rejected.

Taper threads work a little differently

On taper threads there's no separate GO and NOT-GO member in quite the same sense. A basic step gauge or a min/max step gauge is used instead: on external threads, the basic step gauge should sit flush with the component face within a single turn when screwed on by hand; on internal threads, the component face should sit flush between the minimum and maximum step limits of a min/max step gauge.

Why manufacturers use go/no-go gauges

A go/no-go gauge gives a fast, repeatable pass or fail decision on the production line, without a full dimensional measurement on every part. It's the standard method for checking threaded and plain features against a reference standard.