Understanding Internal Clearance: CN, C3 and When It Matters

What is internal clearance?

Internal clearance is the total distance one bearing ring can move relative to the other — radially or axially — before any load is applied. It is a tiny, built-in “play” between the rings and the rolling elements, measured in micrometres.

This play is not a defect. A bearing with zero clearance at room temperature would seize in operation, because clearance shrinks once the bearing is mounted and running.

Why clearance shrinks in operation

Three effects eat into the initial clearance:

1. Interference fits. Pressing the inner ring onto a shaft expands it; pressing the outer ring into a housing compresses it. Tight fits can remove a large share of the initial clearance.
2. Temperature differences. The inner ring usually runs hotter than the outer ring (heat comes from the shaft and the bearing itself). The hotter inner ring expands more, closing the gap.
3. Load. Elastic deformation under load slightly changes the contact geometry.

What matters is the operating clearance — what remains after all three effects. Too little operating clearance causes heat, noise and premature failure; too much causes vibration and poor running accuracy.

The clearance classes

ISO 5753 defines clearance groups for radial bearings. For deep groove ball bearings the common ones are:

Class	Clearance	Typical use
C2	Smaller than normal	High running accuracy, low noise, loose fits
CN	Normal	The default — most applications
C3	Greater than normal	High temperature, high speed, tight fits on both rings
C4	Greater than C3	Very hot environments, heavy interference fits
C5	Greater than C4	Special cases, e.g. severe thermal gradients

A designation without a clearance suffix means CN: 6210 has normal clearance, 6210/C3 has C3.

When to choose C3

Pick C3 instead of CN when one or more of these apply:

• Both rings have interference fits (tight shaft and tight housing)
• Large temperature difference between inner and outer ring — motors, pumps handling hot media, bearings near heat sources
• High speed, where centrifugal growth and friction heat expand the inner ring
• Long shafts where thermal expansion in length must be absorbed

C3 is by far the most common “special” clearance — many distributors stock C3 versions of popular sizes (like 6210/C3) right alongside the CN version.

When not to oversize clearance

More clearance is not a free lunch. Excessive clearance reduces the loaded zone of the bearing, concentrates stress on fewer rolling elements, and increases vibration and noise. If your application has standard fits and normal temperatures, CN is the right choice — do not “upgrade” to C3 without a reason.

Rules of thumb

1. Standard fits, normal temperature → CN (no suffix).
2. Tight fits on both rings, or inner ring much hotter than outer → C3.
3. Very hot or heavy interference → C4, and check with the manufacturer’s calculation tools.
4. Precision spindles with light, controlled preload → sometimes C2 — but that is specialist territory.

The Bearing Selection guide asks about clearance as its final question — when in doubt there, CN is the safe default.