How do I check threads with a go/no-go thread gauge?

How does the go/no-go principle work?

A thread gauge does not report a thread in millimetres; it decides whether the thread lies within the permitted tolerance. It has two sides: the go side (GO) must screw on fully, while the no-go side (NOGO) must not engage, or only by a turn or two.

The go side embodies the maximum-material limit and confirms that the mating part will assemble later. The no-go side embodies the minimum-material limit and catches threads that are too loose or worn oversize. Only when both conditions are met is the thread accepted.

Plug or ring - which gauge for which thread?

Whether you check an internal or an external thread decides the gauge design. Internal threads (nuts, tapped holes) are checked with a thread plug gauge, external threads (bolts, screws) with a thread ring gauge or a snap gauge.

• Plug gauge: the GO end is long and should run through, the NOGO end is short and marked (often with a red groove).
• Ring gauge: the GO ring has no groove, the NOGO ring carries a circumferential groove to tell them apart.
• Snap gauge: faster in batch inspection because GO and NOGO are checked in one pass.
• Each nominal size and tolerance class needs its own dedicated gauge.

Which tolerance classes and markings apply?

Metric ISO threads to DIN 13 are classified by tolerance fields. For internal threads 6H is the standard, for external threads 6g. The gauge must match the workpiece tolerance class, otherwise the result is meaningless.

A gauge designation names the nominal diameter, pitch and tolerance field, for example M10 x 1.5 - 6H. DIN ISO 1502 defines the profile, limit sizes and inspection rules of the gauges themselves, while DIN 13 defines the thread dimensions of the workpiece.

How are thread gauges maintained and verified?

Thread gauges are hardened precision tools that wear slightly with every check. Even light wear on the GO side skews the result, so cleaning, gentle handling and regular calibration are mandatory.

• Never force it on with tools - the go side must engage by hand.
• Clean and lightly oil after use to prevent corrosion and pick-up.
• Store separately and protected so the flanks are not damaged.
• Have it calibrated at defined intervals and document the results.

Frequently asked questions