Internal clearance: CN, C3, C4 explained
Radial internal clearance is one of the most misunderstood parts of a bearing number — a small letter-and-digit suffix that decides whether a correctly-sized bearing runs cool for years or preloads itself and cooks in a week. It is worth understanding, because it is the single most common reason a "same-size" replacement fails.
What clearance actually is
Radial internal clearance is the total distance one ring can move relative to the other, radially, before the bearing is fitted to anything. Hold an unmounted deep groove ball bearing and rock the outer ring against the inner — that tiny play is the clearance. It exists on purpose, and the amount is chosen to suit the fits and the operating temperature.
The clearance classes
| Code | Meaning | Relative to normal |
|---|---|---|
| C2 | Reduced clearance | Less than normal |
| CN (or no code) | Normal clearance | Standard |
| C3 | Greater than normal | More |
| C4 | Greater than C3 | More still |
| C5 | Greater than C4 | Most |
"CN" is the default and is usually left off the number entirely — a plain 6204 is normal clearance, a 6204 C3 has extra. The values are defined by ISO 5753 and depend on bore size (a bigger bearing has a bigger clearance range for the same class).
Why clearance disappears after mounting
The clearance stamped on the box is not the clearance the bearing runs with. Two things eat into it:
- Interference fits. Press the inner ring onto an oversized shaft and the ring expands, shrinking the clearance. A tight housing fit does the same to the outer ring. A large interference can consume most of a normal-clearance bearing's play before it ever turns.
- Temperature. In service the inner ring usually runs hotter than the outer ring. It expands more, and the difference closes the clearance further. A rough rule: every 5 °C of inner-to-outer temperature difference noticeably reduces running clearance.
The goal is that after the fit and the heat, the bearing settles at a small positive operating clearance — near zero but not negative.
What happens when you get it wrong
If the operating clearance goes negative, the bearing preloads itself: the rolling elements are squeezed between the raceways even with no external load. Friction and heat climb, which expands the inner ring more, which increases the preload — a thermal runaway that ends in a blue, seized bearing. This is exactly the failure you get when someone replaces a 6210 C3 with a standard 6210 on a shaft with a heavy press fit and an application that runs hot. The size was identical. The clearance was not.
Too much clearance has milder but real consequences: more noise and vibration, less accurate shaft location, and under some load patterns a shorter life because fewer rolling elements share the load.
When to specify C3 or C4
Choose greater-than-normal clearance when the application will consume more of it:
- Heavy interference fits — large motors, shafts fitted with adapter sleeves.
- High temperature or large temperature gradients — electric motors, hot fans, machinery near ovens.
- Roller bearings under heavy load — spherical and cylindrical roller bearings are very sensitive to clearance and are often specified C3 as standard for that reason.
Choose reduced clearance (C2) only where you need extra stiffness and can guarantee the fits and temperatures won't close it up.
The special case: tapered-bore bearings
Self-aligning and spherical roller bearings with a tapered bore ("K" suffix) are mounted by driving the inner ring up an adapter sleeve. As it goes up the taper, the ring expands and the clearance reduces by a measured amount during installation. For these, clearance is not just selected — it is set on the bench, by how far you drive the ring. That is why a tapered-bore version is never a drop-in swap for a straight-bore one.
The takeaway for cross-referencing
When a table says an SKF, NSK or FAG bearing of the same number are interchangeable, that is true dimensionally — but the clearance class must match on both sides. A 6206 and a 6206 C3 are the same size and a different part. Match the suffix, and when in doubt, keep the clearance the original equipment specified.
General engineering reference only. Confirm the required clearance class and the resulting operating clearance for your fits and temperatures with the bearing manufacturer.