Internal clearance

Internal clearance is the total distance one bearing ring can be displaced relative to the other without external load — radially (radial clearance) or axially. It is measured in the unmounted state and graded for radial bearings into standardized classes: C2 (less than normal), CN (normal), C3, C4 and C5 (progressively larger), per ISO 5753.

Press fits and temperature differences between the rings reduce the clearance in service, which is why bearings with interference fits or hot-running inner rings — electric motors, for example — are often ordered in C3. When replacing a bearing, the clearance class in the suffix must always be matched; fitting CN instead of C3 can cause preload, overheating and early failure.