Internal clearance is the total radial movement of one ring relative to the other, measured before the bearing is mounted and without load. It is the smallest number on the drawing and the one that decides whether a bearing runs cool for ten years or fails in six months — because fits, heat and speed all consume the clearance the bearing was built with.
A bearing is not built with zero play. If it were, the interference fit on the shaft plus the temperature difference between shaft and housing would close the gap entirely and the bearing would run preloaded — hot, noisy, and short-lived. So the factory builds in a defined amount of clearance, and ISO 5753-1 divides that amount into groups.
Only two of the six groups matter in ordinary purchasing: CN (normal, the default when nothing is written) and C3 (greater than normal, the standard for electric motors, fans and pumps). C2 and C4 exist for the tails of the distribution — very tight fits or very hot running. C1 and C5 are special order.
The exact micrometre values depend on bore diameter, so they live in the ISO tables rather than on this page: see the tolerance reference and the individual bearing pages, which carry the clearance table for that size.
| Group | Relative clearance | When it is specified | Availability |
|---|---|---|---|
| C1 | Smallest | Precision instruments where slight preload is intended | Special order |
| C2 | Less than normal | Light fits, low noise, small temperature rise | Common in small sizes |
| CN (C0) | Normal — the default | General engineering with rotating inner ring load | Universal |
| C3 | Greater than normal | Electric motors, fans, pumps, heavy press fits | Universal |
| C4 | Greater than C3 | High temperature, very heavy interference fit | Common in large sizes |
| C5 | Greatest | Extreme heat, large shafts, preload must be avoided | Limited |
Rare, special-order clearance for instrument bearings that should run marginally preloaded.
Read →For light interference fits where heat will not close the gap.
Read →The group you get when the drawing says nothing.
Read →What to specify when heat or a heavy fit would close a CN bearing.
Read →For positions where even C3 would close up at operating temperature.
Read →The last stop before a special clearance, for extreme heat on large bearings.
Read →Nothing. C0 is not an ISO symbol; it is a common shorthand for the normal group, which ISO 5753-1 designates CN. When a drawing says C0, read CN.
A motor has an interference fit on the shaft, a temperature gradient between rotor and housing, and often a preloaded spring washer. All three consume clearance, so motors are built with C3 to leave running clearance under load.
Slightly, at low speed and light load. In practice a C3 bearing in a motor is not audibly different from CN; noise problems come from fits, alignment and grease, not from one clearance group.
Yes, with a dial gauge and a light preload, or with feeler gauges for larger bearings. The measurement is only valid before mounting — once the bearing is on the shaft, the fit has already consumed part of it.
Tell us the application — speed, load, temperature and environment — and we will name the seal, clearance, class and cage that fit it.