Understanding 6g and 6H Thread Tolerance Classes

Understanding 6g and 6H Thread Tolerance Classes

Interchangeability depends on the tolerance band, not the nominal diameter. When an M10 bolt will not enter an M10 nut, the cause is usually a wrong class, not a wrong size. The 6g and 6H markings are the two most common metric bands, and reading them correctly matters for drawing review and reject claims.

Three pieces of information

A thread band has three parts: the tolerance grade number, the fundamental deviation letter and an optional length of engagement. Lower case marks external threads and upper case internal threads; a smaller number means a tighter grade; the letter fixes the position of the band. So 6g is an external thread at grade 6 in position g, and 6H an internal thread at grade 6 in position H.

The pairing leaves clearance that ordinary machining can hold; coating needs 6e, 6f or 7H.

Coating versus thread accuracy

Electroplated zinc at five to eight micrometres adds ten to sixteen micrometres across the pitch diameter, while the whole 6g band on an M10 thread is only about one hundred micrometres wide. The margin is consumed and the go gauge may no longer pass. Three routes exist: produce to 6e or 6f before plating, re-thread after plating, or accept on the coated thread. Re-threading is worst above grade 10.9: it exposes fresh steel and defeats the de-embrittlement bake.

Using go and no-go gauges

The rule is simple: the go gauge must pass, the no-go must not. The go gauge proves the thread stays inside maximum material; over two turns on the no-go means the pitch diameter is undersize. Gauges are turned by hand and they wear, so periodic calibration is required. Two errors are common: forcing the no-go gauge to the bottom and rejecting good parts, or using an overdue gauge and passing bad ones.

Conclusion

Thread class must be agreed at drawing stage, especially where plating is involved. Zhuhai Jiali Hardware (FASTKING) supplies precision screws and high-strength bolts to 6g, 6e or customer bands with inspection records. See the screw calculator directory or contact us.