C3 vs C4 Clearance: Which One Survives Your Heat?

C3 and C4 are not quality grades — they are allowances for clearance that will be consumed by the interference fit and by thermal growth. Choosing between them is arithmetic: estimate how much clearance disappears at operating temperature, and pick the group that still leaves some.

40–80 °C vs 80–120 °CWorked exampleArithmetic, not opinion

Short answer. C3 is correct for most motors, fans and pumps, where the shaft runs 40–80 °C hotter than the housing. C4 is correct for dryer rollers, kilns and heavy press fits, where the difference exceeds 80 °C. If you cannot estimate the difference, measure the housing and shaft temperatures with a contact thermometer before ordering.

What consumes clearance

Two things, both predictable. First the fit: an interference of 20 µm on the shaft expands the inner ring and removes roughly 15–18 µm of radial clearance. Second the heat: steel expands by about 11 µm per metre per °C, so a 100 mm bore inner ring running 60 °C hotter than the outer grows about 66 µm more in circumference — which is consumed as clearance.

Clearance groups and the conditions they assume
Group	Assumed temperature difference	Assumed fit	Typical application
CN	Under 40 °C	Light to normal	General machinery
C3	40–80 °C	Medium interference	Motors, fans, pumps
C4	80–120 °C	Heavy interference	Dryer rollers, kilns, large fans
C5	Above 120 °C	Very heavy, or both rings	Kiln cars, furnace rollers

A worked example: 100 mm bore dryer roller

Shaft at 180 °C, housing at 70 °C, temperature difference 110 °C. The inner ring grows about 11 µm per 100 mm per 100 °C, so roughly 121 µm at 110 °C; the outer ring, cooler, grows far less. Add an interference fit of 30 µm, which removes about 25 µm of clearance. Total consumption is well beyond the C3 band even before the operating load — which is why this position takes C4 or C5, and why a C3 bearing here fails by preloading within weeks.

The symptoms of getting it wrong

Too tight (C3 where C4 was needed): the bearing runs hot, grease darkens early, and the raceway shows a polished, distressed band — classic preload failure.
Too loose (C4 where C3 was needed): vibration at light load, a rumbling tone at low speed, and cage wear from elements skidding instead of rolling.

Practical rule: Match the clearance to the measured temperature difference, not to what the previous supplier shipped. A like-for-like replacement inherits the old specification — including its mistakes.

Frequently asked questions

Is C4 better than C3?

No. C4 is looser, which is correct for higher temperatures and heavier fits and wrong for everything else. Excess clearance lets the rolling elements skid and loads the cage.

How do I measure the temperature difference?

A contact thermometer on the housing near the bearing and on the shaft as close to the bearing as possible, after the machine has reached steady state. Two readings answer the question.

Can I use C3 in a kiln car?

Almost never. A kiln car shaft typically runs more than 120 °C hotter than the housing, which requires C4 or C5.

Does C4 need a different lubricant?

Usually yes — C4 implies high temperature, which means a high-temperature grease and often a steel or brass cage rather than polyamide.

Ready to price this?

Send the model list and quantities — we reply with brand options, price and lead time within 30 minutes on working days.

Send your model list Check interchange