| Tolerance class, tap |
Tolerance, internal thread (nut) |
Application |
| ISO |
DIN |
ANSI
BS |
| ISO 1 |
4 H |
3 B |
4 H |
5 H |
|
|
|
Fit without allowance |
| ISO 2 |
6 H |
2 B |
4 G |
5 G |
6 H |
|
|
Normal fit |
| ISO 3 |
6 G |
1 B |
|
|
6 G |
7 H |
8 H |
Fit with large allowance |
| - |
7 G |
- |
|
|
|
7 G |
8 G |
Loose fit for following
treatment or coating |
Tap tolerance vs. tolerance on internal thread (nut)
Normal tap tolerance is ISO 2 (6H), which generates an average quality fit between screw and nut. Lower tolerance (ISO 1) generates a fine fit without a gap on the flanks between screw and nut. Higher tolerance (ISO 3) generates a rough fit with a large gap. This is used if the nut will be coated or if a loose fit is preferred.
Between tolerances 6H (ISO2) and 6G (ISO3), as well as between 6G and 7G, there are also taps with tolerance 6HX and 6GX. “X” means the tolerance is outside the standard and is used for taps working in high strength material or abrasive material such as cast iron. These materials do not cause oversize problems and higher tolerance can be used in order to increase tool life. The tolerance width is equal between, for example, 6H and 6HX.
Forming taps are usually produced with a 6HX or 6GX tolerance.
Pipe threads refer to the following standards
G threads to ISO 228-1. One class for internal thread (tap)
Rc and Rp threads to ISO 7-1.
NPT and NPSM to ANSI B1.20.1.
NPTF and NPSF to ANSI B1.20.3
Tolerance positioning
Tolerance width on a tap designed for a specific tolerance is much smaller than tolerance width on the finished thread.
The tap tolerance is positioned so the tap cuts a correct thread from the start. When the tap is used it wears gradually and at one point the tap is too small to cut a correct thread, it becomes smaller than the lower tolerance of the GO gauge.
The optimum would have been to position the tap tolerance at the upper part of the internal thread area but then the risk is too big that the tap cuts an oversized thread, above the upper limit of the NO-GO gauge.
The “X” tolerances, used for materials that do not cause oversize problems, are positioned higher. The result is a longer tool life due to that more wear can be accepted before the tap cuts a too small thread.
DIN tolerances
 |
Thread tolerance classes |
| Tap tolerances zones |
ANSI tolerances
| |
Thread tolerance classes |
| Tap tolerances zones |