Industrial MRO Reference Library · Technical Reference

Bearing Internal Clearance — C2, CN, C3, C4

Radial internal clearance is the small amount of play between the rings and rolling elements of an unmounted bearing. This sheet explains the C2, CN, C3 and C4 classes and when C3 is specified.

Ref MRO-0004ISO 5753-1

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Radial internal clearance is how far one ring can move radially relative to the other in an unmounted bearing. ISO 5753-1 defines groups of clearance for each bore size range. Normal clearance (CN) has no suffix on most bearings. C2 is less than normal. C3 is greater than normal. C4 and C5 are greater again. The actual values in micrometers depend on bearing type and bore size and are listed in ISO 5753-1 and in each maker's catalog.

How to identify

Read the suffix after the basic number and any closure codes. "6205-2RS C3" has C3 clearance. No clearance suffix means normal (CN). Some brands mark clearance as "/C3," "C3" or just "3" in part of the suffix. Check the brand's designation guide.

Clearance groups

Group	Suffix	Clearance compared with normal	Typical reason to use it
Group 2	C2	Less	Precision or low-noise uses where little play is wanted
Normal	CN or none	Normal	Most general applications with standard fits
Group 3	C3	Greater	Electric motors, interference fits, inner ring hotter than outer
Group 4	C4	Greater than C3	Heavy interference fits, high temperatures, some vibrating equipment
Group 5	C5	Greater than C4	Special cases, per the equipment maker

Why clearance changes in service

When a bearing is pressed onto a shaft with an interference fit, the inner ring expands and some clearance is lost. When the inner ring runs hotter than the outer ring, it expands more and takes up more clearance. A bearing with too little operating clearance runs hot, its lubricant breaks down and it fails early. Extra initial clearance (C3 or C4) leaves enough room after these effects.

Key points

Electric motor bearings are commonly specified with C3 clearance. Replace like with like.
Do not change clearance class to fix a noise or fit problem without checking with the equipment maker. More clearance can mean more noise and vibration in some machines.
Clearance classes differ by bearing type. A C3 deep groove ball bearing and a C3 spherical roller bearing do not have the same micrometer values.
Mounted clearance, not unmounted clearance, is what matters in service. Fits, temperatures and installation method all affect it.

Common mistakes

Dropping the C3 suffix when quoting or cross-referencing a motor bearing.
Assuming "no suffix" means "no clearance." It means normal clearance.
Using a C2 bearing in a motor because it felt tighter and seemed better.
Comparing clearance values across different bearing types.

Governing standards: ISO 5753-1 (radial internal clearance). Manufacturer catalogs for values and designations.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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