The standard also distinguishes between different thread classes depending on tolerance and allowance:
| External threads | Internal threads | Type of fit | Examples | |
| 1A | 1B | loose fit | Rigging gear, turnbuckles | |
| 2A | 2B | normal fit | Flanges, ordinary bolted connections | |
| 3A | 3B | tight fit | Socket head cap screws | |
| NC5 HF | interference fit | |||
| NC5 CSF | interference fit | |||
| NC5 ONF | interference fit | |||
| NC5IF | interference fit | |||
| NC5INF | interference fit for materials with hardness less than 16 = HB |
The classes shall normally not be used to accommodate the thickness = of the coating. Further details on tolerances are given
Default metric fastener thread tolerance class is "medium", corresponding to 6H/6g. The 6H/6g fit is the standard ISO tolerance class for general use.
B1.1 internal and external thread tolerance class 2B/2A is essentially equivalent to ISO thread tolerance class and fits 6H/6g. Imperial tolerance class 3A is approximately equivalent to ISO tolerance class 4g6g, though class fit 3B/3A is approximately equivalent to ISO class fit 4H5H/4h6h. For full details, see ISO 965-1, Sects. 5.2, 7, and 12.