These gauges come with a certificate of calibration traceable to NIST that states they’ve passed a test for accuracy. They allow you to inspect internal threads on parts. All have a go gauge on one end and a no-go gauge on the other end. A taperlock handle lets you quickly replace worn gauges.

Check the minimum internal thread size with the go gauge end, and check the maximum internal thread size with the no-go gauge end. A part is within tolerance if it’s easy to thread the go gauge into the part but difficult to thread the no-go gauge more than two turns.

Threads have different classes of fit. Thread fit designates the tightness or looseness of mating threads. For example, inch threads with a 3B fit have tighter tolerances than threads with a 2B fit.