Bearing clearance explained: C2, CN, C3 and C4

What radial internal clearance means

For an unmounted radial bearing, radial internal clearance is the total distance one ring can be moved radially relative to the other from one extreme position to the other. It is internal movement between the rings and rolling elements, not looseness between the shaft and bearing bore and not the housing fit.

This guide concerns radial internal clearance. Axial clearance, preload and clearance adjusted during mounting are separate engineering topics.

C2, CN, C3 and C4 from smaller to larger

The order is C2, CN, C3, C4. C2 is smaller than normal, CN is the normal group, C3 is greater than normal and C4 is greater than C3. Each class is a range rather than one universal micrometre value; the limits depend on bearing type and bore range.

Greater does not mean better. C3 is not a precision, material, seal or quality rating. Normal clearance is often omitted from the complete designation, and some maker literature also uses C0 terminology, so verify the manufacturer's convention rather than inferring it from an incomplete marking.

Initial, mounted and operating clearance

The clearance class describes the bearing before the machine's mounting and thermal effects are fully applied. Interference on the inner ring can expand it, while interference around the outer ring can compress it. Either can reduce the internal clearance after mounting.

The operating clearance can differ again once temperatures stabilise. If the inner ring becomes hotter than the outer ring, its greater expansion commonly reduces radial clearance. Other materials and thermal arrangements can behave differently, so ambient temperature alone does not decide the class.

Is C3 better than CN?

No. C3 starts with more radial internal clearance and is appropriate only where the bearing selection, fits and operating conditions require it. It may be selected to leave an appropriate operating clearance after mounting and temperature effects; it is not automatically correct for every hot, fast or heavily loaded machine.

Why too little or too much clearance matters

Too little or negative operating clearance can increase friction and heat and shorten service life. Excessive clearance can worsen noise, vibration, load distribution, running accuracy or rigidity depending on the bearing and application. An unmounted bearing's feel is not a reliable acceptance test for these small, defined clearance ranges.

When not to guess

Replace like for like using the full marking or OEM reference where the machine design is unchanged. Stop and verify when the suffix is unreadable, the shaft or housing has been repaired, fits or materials changed, operating temperature, speed or load changed, both rings use interference, or the assembly is safety- or performance-critical.

A marking such as 6203-C3 combines the 6203 basic designation with a greater-than-normal radial internal-clearance class. A bare 6203 may mean normal clearance under a maker's system, but the complete manufacturer product record remains authoritative.