Choosing a Radial Internal Clearance Class (C2, CN, C3, C4) from Fit and Temperature: What the Evidence Settles and What You Must Take from ISO 5753-1

The class you specify is the clearance left after fitting and heating

A bearing's radial internal clearance is measured on the bench, before it is mounted. Press an inner ring onto an interference shaft fit and the ring expands, which reduces that bench clearance¹¹.

Run the bearing and let the inner ring sit hotter than the outer ring, and the differential expansion takes more of it away². The class printed on the box is therefore a starting point, not the operating condition.

This is why a drawing or quote that names C3 without stating the shaft and housing fits and the expected ring temperatures is not yet a specification. You cannot tell from the class alone whether the bearing will run with clearance or without it.

Ask for the fit classes and the temperature basis alongside the clearance designation, then check that the residual clearance is positive at operating temperature¹¹.

One supplier's worked estimate puts the scale of the loss in perspective: a k6 fit on a 30 mm shaft removes about 12 µm, and a 50 °C ring differential removes another 5–10 µm²⁵. Treat those as that source's estimate for that case, not as a rule for your bore size.

Rule out any class that would run at zero or negative clearance

If the fit and the heat together consume more clearance than the bearing was made with, the rolling elements become preloaded. Friction rises, the bearing runs hotter, the rings expand further and the clearance goes more negative still.

One bearing supplier describes this as a thermal runaway loop that ends in rapid failure once the bearing exceeds what the lubricant can take¹¹. Another states the same outcome more briefly: too little clearance causes excessive friction, rapid heating and seizure¹⁸.

That is the failure mode you are ruling out when you choose a class. It is not a wear-out mechanism you can inspect for later; it is a running condition that has to be avoided by design.

If your fit and temperature estimate leaves little or no residual clearance on paper, the class is too tight for the duty, whatever the catalogue says. What the sources do not give is a threshold residual clearance to design to, or a formula that turns your fit and temperature inputs into a clearance loss figure.

Those have to come from the bearing supplier or the governing standard.

Let the load type set the fits before you touch the class

The fit is what consumes clearance, so the fit decision comes first. The ring that carries the rotating load should have an interference fit; the degree of interference depends on bearing type, size and operating conditions¹.

When the load direction is indeterminate, particularly with heavy loads, one bearing maker recommends an interference fit on both rings¹. Two interference fits remove clearance from both sides of the bearing, so the class has to be chosen after that decision, not before it.

Housing tolerance class follows the same logic. For a stationary outer ring load, H6, G6, J6 or J7 are the usual choices; for an indeterminate or rotating outer ring load, K7 or M7^10,15.

On the shaft side, light-to-normal rotating inner ring loads take js6, k6 or m6, while heavy or impact loads move to n6, p6 or r6¹⁵. Heavy or impact loads also call for a larger internal clearance than standard¹⁹.

Work through the load case first: which ring rotates relative to the load, and how heavy is it. The answer tells you how much interference you are committing to, and therefore how much bench clearance the fit will eat before temperature is even considered.

Correct for the ring temperature differential in the right direction

A hotter inner ring expands and takes clearance away; a hotter outer ring expands the housing seat and adds clearance². The direction matters more than the magnitude, because it decides whether you step up a class or stay where you are.

Where a temperature differential between inner and outer rings exists, one bearing maker's motor example specifies a clearance larger than Normal, C3⁴. Where the outer ring runs hotter, the thermal effect works in your favour and no thermal allowance is needed.

Large bearings have a separate housing rule tied to the same physics. Above 250 mm outside diameter, if the outer ring runs more than 10 °C hotter than the housing, G7 replaces H7, and F7 replaces G7¹⁰.

That is a housing tolerance correction, not a clearance class change, and it applies to the large-bearing case the source states. One bearing maker gives a relation for how much of the bearing-to-housing temperature difference appears at the fitting surfaces: roughly 0.10 to 0.15 times that difference¹⁷.

It is a starting input for estimating interference loss, not a clearance correction coefficient. No source in this set gives a µm-per-°C clearance change figure, so the thermal allowance itself has to be confirmed with the supplier.

Pick the starting class, then check the exceptions

Normal (CN) is the sensible starting point when temperatures are stable, fits are light and you need precise shaft guidance⁹. C2 is tighter still and suits minimal-runout work at controlled temperature with light fits, such as instruments and measuring equipment; it has little margin against an interference fit or an unexpected temperature rise²⁵.

C3 is the step up for consistently high operating temperature, heavy interference fits or speeds above the standard range⁹. C4 is the next step again, reserved for extreme temperature, large shaft-to-housing thermal differentials, thin-walled housings or severe misalignment⁹.

The exception that catches buyers is bearing type. For cylindrical roller bearings, Normal clearance is usually preferred over C3, because those bearings already run with tighter fits and higher initial clearance¹².

A C3 default carried across from a motor specification to a cylindrical roller bearing is therefore the wrong starting point for that bearing type. One supplier states C3 as the industry standard for electric motors and C4 for steel mill work rolls, kiln rollers and paper dryer rolls²⁵.

Another calls C3 the default for many common industrial motors and gearboxes⁹. Those are supplier positions on common practice, not standard requirements.

Check the bearing type and the actual fit and temperature case before adopting them.

What one class up or down actually costs you

The penalty directions are consistent across the sources even though their sizes are not quantified. Go one class too tight and you risk the heating and seizure path described earlier¹⁸.

Go one class too loose and the bearing loses shaft guidance: vibration rises and the load distribution across the rolling elements becomes uneven¹⁸. Clearance also affects noise, deflection, heat stress and fatigue life, so the choice is not confined to the seizure question².

The noise and vibration answer depends on bearing type rather than following a single rule. One supplier claims C3 clearance helps reduce vibration and compensate slight misalignment¹⁶.

For cylindrical roller bearings, by contrast, Normal clearance with a normal fit keeps the loaded zone appropriate and reduces noise and smearing risk, and an upper-C2 range is offered to cut noise further¹². If low noise is your driver, the class that helps depends on which bearing you are specifying.

No source in this set quantifies the fatigue-life or load-capacity change from a one-class step. If your decision hinges on that number, it has to come from the bearing supplier's calculation, not from a rule of thumb.

Verify the delivered clearance before you accept the lot

A clearance check on the bench does not read the same as the catalogue value, because the measuring load itself opens the clearance. One bearing maker's table requires the measured value to be corrected by adding the tabulated amount for the measurement load before comparing it to the class range²¹.

Without that correction, the reading is larger than the true clearance, so a bearing that is tighter than the class you ordered can pass the check.

• ✓Confirm the drawing or purchase order states the clearance class and the fit classes together, not the class alone.
• ✓Obtain the class-by-bore numerical range from ISO 5753-1 or the supplier's catalogue before writing the acceptance limit.
• ✓Ask the supplier for the measurement-load correction values that apply to your bore size and class.
• ✓State whether the clearance is to be checked before or after mounting, and correct for the fit if it is checked after.
• ✓For cylindrical roller bearings, confirm Normal clearance is the intended starting point rather than a carried-over C3 default.

Where the sources disagree

The default class for motor-type duty is the main open question, and the only published clearance figures in this set come from a single supplier page. Read both sides before you fix the class in a specification.

Before you request a quote

• The evidence leaves this open — put it to the supplier: no complete numerical clearance table by class and bore diameter (C2, CN, C3, C4, C5) from a governing standard or authoritative technical source is in this evidence set; obtain it from ISO 5753-1 or the bearing supplier.
• The evidence leaves this open — put it to the supplier: no readable formula for computing clearance reduction from interference fit and thermal differential is available; the source formulas are images. Ask the supplier to calculate the loss for your bore, fit and temperatures.
• The evidence leaves this open — put it to the supplier: no authoritative µm-per-°C or speed-band clearance change rule is given; do not apply a coefficient from this article.
• The evidence leaves this open — put it to the supplier: no direct mapping table from a specific shaft or housing fit (k5, m5, j5, H7, J7, K7) to a recommended clearance class exists in this evidence; the fit rules are argued in prose only.
• The evidence leaves this open — put it to the supplier: no quantitative fatigue-life, load-capacity or heat-generation penalty for a one-class change is given; request it from the supplier if the decision depends on it.
• The evidence leaves this open — put it to the supplier: no measurement-load correction values or per-class acceptance criteria are shown; obtain them from the supplier or the standard before writing an acceptance clause.
• Request a written quote with price basis, MOQ, tooling, payment terms and production lead time.
• The evidence leaves this open — put it to the supplier: the C5 class is not covered by any source in this set.
• Request the complete specification sheet with units, test conditions and configuration assumptions.