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.

No-go gauges check the minimum 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.