A certificate of calibration traceable to NIST states that these gauges have passed a test for accuracy. They’re for checking a part’s external threads to ensure they fall within tolerance.

A part is within tolerance if it’s easy to thread a go gauge onto the part but difficult to thread a no-go gauge more than two turns.

Go gauges check the maximum external thread size.

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

For technical drawings and 3-D models, click on a part number.