Guide
Bearing clearance explained: C2, CN, C3 and C4
What the C3 suffix on a bearing means, how internal radial clearance changes when the bearing is mounted and warms up, and when C3 is the right choice.
The suffix C3 is one of the most common additions to a bearing number — 6205-2RS-C3, 6305-C3, NU210-E-C3. It is also widely misunderstood. C3 is not a higher grade or a tighter tolerance. It describes how much internal radial clearance the bearing has before it is mounted.
What internal radial clearance is
Internal radial clearance is the total distance one ring can move radially relative to the other when the bearing is not mounted and not loaded. Every rolling bearing needs some play: too little and the bearing preloads itself, runs hot and fails early; too much and it runs noisily and loads fewer rolling elements.
Clearance classes are standardised in groups. The exact micrometre ranges depend on bearing type and bore size, so a C3 in a 25 mm deep groove ball bearing is a different number of micrometres from a C3 in a 200 mm spherical roller bearing.
| Class | Meaning | Marked as |
|---|---|---|
| C2 | Less than normal | C2 |
| CN | Normal | Usually no suffix (sometimes CN) |
| C3 | Greater than normal | C3 |
| C4 | Greater than C3 | C4 |
| C5 | Greater than C4 | C5 |
Because normal clearance is the default, a bearing with no clearance suffix — a plain 6205 — is CN.
Why clearance changes in service
The clearance in the catalog is the unmounted value. Two things reduce it once the bearing is running:
- Interference fits. When the inner ring is pressed onto a shaft with an interference fit, it expands slightly. When the outer ring is pressed into a tight housing, it contracts. Both eat into the clearance.
- Temperature difference. In most machines the inner ring and shaft run warmer than the outer ring and housing. The inner ring expands more, reducing clearance further.
What is left after mounting and at operating temperature is the operating clearance, and that is what matters. Choosing a clearance class is really about making sure the operating clearance stays slightly positive.
When C3 is used
C3 is common wherever fits are tight or the shaft runs hot:
- Electric motors. Interference fits, continuous duty and heat from the windings make C3 the default for many motor bearings.
- Heavy interference fits on rotating inner rings with high loads.
- Hot shafts, for example in fans handling warm air, dryers or rolls heated from inside.
- Spherical and cylindrical roller bearings in heavy industry, where C3 is often specified as standard for the application.
- Tapered-bore bearings mounted on adapter sleeves, where the clearance reduction during mounting is used to set the fit.
C2 appears where very low noise or precise radial location matters and fits are light. C4 and C5 are used in very hot or very tightly fitted applications such as vibrating screens or some rolling mills.
What C3 does not mean
- Not a quality grade. C3 bearings are not better or worse than CN; they are different.
- Not precision. Running accuracy is set by tolerance classes such as P6 or P5 — see the suffix dictionary.
- Not a closure. 6205-2RS-C3 has seals and C3 clearance; the two suffixes are independent. See sealed vs shielded bearings.
- Not universally interchangeable with CN. In a lightly fitted, cool application a C3 bearing will run with more play than intended; in a hot motor a CN bearing may lose its clearance entirely.
Replacing a bearing: match the clearance
The simplest rule for maintenance is to replace like with like. If the old bearing was marked C3, fit a C3. If it had no clearance mark, fit CN, unless the equipment manufacturer specifies otherwise. Changing clearance class is an engineering decision that depends on fits, speed, load and temperature — not something to decide from the parts shelf.
If the original marking is gone, the equipment manual or spare parts list usually states the full designation. Motor nameplates and motor data sheets often list the bearing numbers including C3.
Clearance in catalog data
In a bearing catalog, clearance is one of the attributes that most often gets lost. Supplier exports put it in the part number, in a description, or nowhere at all. Store it as its own field (internal_clearance: C2 / CN / C3 / C4 / C5) and keep “not stated” separate from “normal” — a blank cell does not prove CN. Our guide on missing versus not applicable explains why that distinction matters.
Apply it
Every size page on the bearing search lists the common clearance variants for that size. To decode a full number such as 6305-2Z-C3, use the bearing number decoder. Cleaning clearance data across a catalog? See normalization or contact us.
Methodology article using synthetic examples only; it states no manufacturer specifications. Independent technical review: not yet assigned. Found an error? Tell us.
