no-go

A no-go gauge is the larger or looser of a pair of limit gauges used to check whether a machined part falls within tolerance. Where a go gauge confirms the part is large or tight enough, the no-go gauge verifies it has not been made oversize or too loose. If a part enters or engages the no-go gauge, the part is scrap or rework.

Limit gauges come in matched pairs. A go gauge sets the lower tolerance limit; a no-go gauge sets the upper limit. For a hole drilled to 10 mm +0.05 / -0.00, the go gauge would be 10.00 mm and the no-go would be 10.05 mm. The part must pass the go gauge smoothly and must not enter the no-go gauge at all. This binary pass-fail check is faster and cheaper than measuring with a micrometer and eliminates operator error from reading an instrument.

No-go gauges are made in two main forms. Plug gauges check internal holes and bores; ring gauges check external diameters and shafts. Both are hardened steel cylinders with the no-go diameter ground to exact size. The no-go end is usually painted red or marked with a groove to distinguish it from the go end. Wear on the no-go gauge is critical: if it becomes worn and oversize, good parts will incorrectly be rejected.

No-go gauges are most common in high-volume production, where the speed of inspection justifies the cost of maintaining a gauge set. In low-volume or job shops, direct measurement is often more practical. No-go gauges deteriorate with use, especially under abrasive conditions or when used on rough as-cast surfaces. Regular calibration against a master gauge is required; many shops establish a replacement schedule, retiring gauges after a fixed production volume.

The term no-go reflects the gauge's function as a barrier: the part should not go into it. This contrasts with go gauges, which the part should freely go into. The distinction is fundamental to British and ISO gauge terminology and appears across all types of limit checking, from thread plug gauges to taper gauges.