What does C3 mean on a bearing, and does it matter?
C3 is one of the most misunderstood markings on a bearing. It is about internal clearance, and using the wrong one can shorten a motor bearing’s life.
Two bearings can be the same size, the same brand and the same seal type, and still not be interchangeable. The difference is often two characters at the end of the number: C3.
Internal clearance, explained
Hold the inner ring of a new bearing and push the outer ring sideways. There is a tiny amount of play between the rings and the balls. That is the internal radial clearance, measured in microns (thousandths of a millimetre).
Clearance is graded into groups:
- C2: less than normal
- CN: normal. This is the default and usually not marked.
- C3: greater than normal
- C4, C5: greater still
For a sense of scale, a 6204 (20 mm bore) has roughly 5 to 20 µm of clearance in the normal group, and roughly 13 to 28 µm in C3.
Why would you want more play?
Because a bearing loses clearance once it is installed and running:
- Tight fits. When the inner ring is pressed onto a shaft, it stretches slightly and the clearance shrinks.
- Heat. In a running motor, the inner ring and shaft run hotter than the outer ring and housing. The inner ring expands more, and the clearance shrinks again.
If too little clearance is left, the balls end up squeezed between the rings. The bearing runs hot, gets noisy and fails early. C3 gives the extra room that motors, high-speed machines and hot-running equipment need.
Where you will find C3
- Electric motors: most motor makers specify C3 deep groove bearings.
- Power tools such as grinders and drills, which run fast and hot.
- Vibrating screens and fans, where spherical roller bearings are often C3 or C4.
- Anything with a heavy interference fit or a hot shaft.
Can I use a normal bearing instead of C3?
Not as a rule. If the original was C3, the machine was designed for it, and fitting a normal-clearance bearing risks the overheating described above. Going the other way, fitting C3 where normal was specified, is less risky but can add noise and vibration in precision equipment.
Replace like with like. If the old bearing says C3, ask for C3.
We keep the popular sizes in both normal and C3 clearance, from SKF, FAG, NTN, NSK and others. If you need C2 or C4, or a mixed requirement, we can source that too.