C3 is the most common bearing internal clearance grade for MRO replacement — but many buyers don’t know when they need it.
What internal clearance means
Internal clearance is the total distance the inner ring can move radially relative to the outer ring before the rolling elements engage. It affects heat management, noise, and bearing life.
Why C3 exists
When a bearing is pressed onto a shaft (interference fit), the inner ring expands. When it’s pressed into a housing, the outer ring contracts. Both reduce the internal clearance. Add thermal expansion from operating heat, and a CN (normal) bearing can run with zero clearance — which causes seizure.
C3 provides extra play to absorb these reductions.
When to specify C3
- The shaft fit is interference (k5, m5, or tighter)
- Operating temperature exceeds 80degC
- Speed exceeds 75% of the bearing’s limit
- The original failed bearing shows blue discoloration (overheating)
- Motor bearings: most electric motor manufacturers fit C3 as standard
How to check your old bearing
Look at the etching on the side face. SKF marks C3 as 6205/C3. FAG marks it as 6205.C3. If the bearing is too damaged to read, check the motor nameplate — if it mentions C3, specify C3 on your replacement order.
If unsure, tell KIHOI the application (motor, pump, gearbox) and we will recommend the correct clearance.