Bearing Internal Clearance: C2, CN, C3, C4 and C5 Explained

Key takeaways

• Internal clearance is the total distance one bearing ring can move relative to the other. It is not the same thing as precision class.
• The standard classes run from C2 (less than normal) through CN (normal), C3, C4 and C5 (largest). The values for each class depend on bearing type and bore size and are defined in ISO 5753-1.
• Catalog clearance is measured on the unmounted bearing. Press fits and temperature differences eat clearance, so the operating clearance is always smaller than the number on the box.
• C3 is not "better" than CN. It is the right choice when mounting fits or heat remove more clearance than a normal application would, and the wrong choice when they do not.
• Wrong clearance shows up in vibration data and bearing temperature long before failure, which is why clearance choices belong in your maintenance records.

What bearing internal clearance actually is

Radial internal clearance is the total radial play between the rings and the rolling elements of an unmounted bearing: push the outer ring fully down, then fully up, and the total travel is the clearance. Axial clearance is the same idea measured along the shaft axis.

Manufacturers group this play into classes. From smallest to largest: C2, CN (normal), C3, C4, C5. A bearing marked with no clearance suffix is CN. The actual micrometre values behind each class are not fixed numbers: they scale with bore diameter and differ by bearing type, and they are standardized in ISO 5753-1 and in every major manufacturer catalog.

For a common deep groove ball bearing with a 50 mm bore (a 6310, for example), the catalog ranges look like this:

Do not confuse clearance classes with precision classes (P0, P6, P5 and up). Precision classes control dimensional and running accuracy. A P5 bearing can have C3 clearance, and a standard precision bearing can be C2. They answer different questions.

Why the mounted clearance is always smaller

Two effects shrink the catalog clearance the moment the bearing goes to work:

1. Interference fits. Pressing the inner ring onto the shaft expands it, and pressing the outer ring into the housing compresses it. As a working rule, roughly 80 percent of the effective interference comes straight out of the clearance.

2. Temperature differential. The inner ring usually runs hotter than the outer ring. The differential expansion removes clearance at roughly α × ΔT × d_m, where α is 12 × 10⁻⁶ per °C for steel and d_m is the mean bearing diameter.

A worked example with real numbers

Take that 6310 on a 50 mm motor shaft with a k5 shaft fit and a loose housing fit:

• Catalog clearance, CN class: 6 to 23 µm. Assume mid range, about 15 µm.
• Effective interference from the k5 fit: about 15 µm. Clearance loss at 80 percent: 12 µm.
• Inner ring runs 5 °C hotter than the outer ring. Mean diameter of a 6310 is about 80 mm. Thermal loss: 12 × 10⁻⁶ × 5 × 80,000 µm = about 5 µm.
• Operating clearance: 15 - 12 - 5 = roughly zero.

Zero or negative operating clearance in a deep groove ball bearing means rising temperature, rising preload and a runaway failure loop. This is exactly why electric motors are routinely fitted with C3: start from the C3 mid range of about 24 µm and the same arithmetic lands at a healthy 7 µm of running clearance.

How to choose the right class

• CN when one ring is clamped, the other slides, and the machine runs near ambient temperature.
• C3 when the inner ring gets a press fit, the machine runs warm, or the manufacturer of the driven equipment specifies it. Most industrial motors ship with C3 as standard.
• C4 and C5 when both rings are fitted with interference, or the temperature differential is large: dryer sections, vibrating screens, traction motors.
• C2 only when the design demands minimal play and loads are light. It is rare in plant equipment.

Whatever you choose, write it down. When a bearing is replaced during a breakdown at 2 a.m., the fitter will install whatever is in the store. If the machine history in your CMMS records the exact designation including the clearance suffix, the right spare goes in. If it just says "6310", you get whatever was cheapest last order. That single missing suffix is a classic hidden cause behind repeat failures and shortened MTBF.

How wrong clearance shows up before failure

Too little clearance shows up as steadily climbing bearing temperature and a rising overall vibration level. Too much clearance shows up as looseness signatures in the vibration spectrum and audible rumble at low speed. Both are visible in ISO 20816 vibration severity readings and in temperature trends well before the bearing lets go, which is what gives predictive maintenance tools something to work with.

Clearance problems also love company: a bearing that was hammered on cold, a shaft seat worn under size, or an overtightened adapter sleeve will each mimic a wrong clearance choice. Good fitting practice matters as much as the suffix on the box, the same way correct bolt torque matters more than the gasket brand.

Common mistakes

• Ordering C3 for everything "to be safe". On a loose fitted, cool running application the extra play just raises vibration and noise.
• Reading clearance as quality. A C4 bearing is not a heavy duty bearing. It is a bearing that expects to lose more clearance during mounting and running.
• Ignoring the housing side. An interference housing fit on the outer ring removes clearance exactly like a shaft fit does.
• Measuring the old bearing after removal and matching the spare to it. Worn clearance is not design clearance.
• Not recording the full designation in the work order, so the next replacement silently changes the clearance class.

Keeping bearing designations, fits and failure history in one place is a job for your maintenance system, not for someone's notebook. A CMMS that holds the machine history and the exact spare part specification per position, like the approach described in our preventive maintenance plan guide, closes that loop. If you want to see how Fabrico records machine history down to component level, book a short demo.

The same trend logic applies one step up the drivetrain: our guide to measuring gearbox backlash turns a dial indicator reading into a wear verdict.

Frequently asked questions

Is C3 clearance bigger or smaller than normal?

Bigger. The scale runs C2, CN (normal), C3, C4, C5 from smallest to largest. C3 is one step more clearance than normal.

Why do electric motors use C3 bearings?

Motor shafts get an interference fit and the inner ring runs hotter than the outer ring. Both effects remove clearance, so the motor needs to start with more of it to end up at a healthy operating clearance.

Can I replace a CN bearing with a C3 bearing?

Mechanically it fits, but the running behaviour changes. On a warm, press fitted position it is often an improvement. On a cool, loose fitted position it adds play, noise and vibration. Match the clearance to the application, not to the shelf.

What is the difference between clearance class and precision class?

Clearance classes (C2 to C5) describe internal play. Precision classes (P0, P6, P5) describe manufacturing accuracy of dimensions and running trueness. They are independent choices.

How do I know which clearance a bearing has?

Read the suffix on the designation: 6310 C3 has C3 clearance, plain 6310 is CN. The actual micrometre range behind the class depends on bore size and bearing type and is listed in ISO 5753-1 and manufacturer catalogs.