Short answer
No, bike wheel bearings are not all the same size. Hubs from different brands and eras use different bearings, and even the front and rear hubs of one wheelset can differ. The reliable way to find the right size is to check the markings on the old bearing, measure it, or look up the hub model with the manufacturer.
How bearing sizes are written
A sealed cartridge bearing is described by three measurements, in millimeters:
- Inner diameter (ID): the hole the axle passes through.
- Outer diameter (OD): the width of the bearing, which sits in the hub shell.
- Width: the thickness of the bearing.
These are often written as ID x OD x width, for example 15 x 28 x 7. Many bearings also have a printed code. Codes like 6902 refer to a standard size, but different manufacturers may add letters for seals or other details (for example “2RS” for a rubber seal). Some bike brands also use their own codes.
Common cartridge sizes
The table lists sizes that are commonly found in bike hubs. It is not complete, and your hub may differ.
| Code | ID x OD x width (mm) |
|---|---|
| 608 | 8 x 22 x 7 |
| 6000 | 10 x 26 x 8 |
| 6900 | 10 x 22 x 6 |
| 6001 | 12 x 28 x 8 |
| 6901 | 12 x 24 x 6 |
| 6902 | 15 x 28 x 7 |
| 6903 | 17 x 30 x 7 |
Other sizes, such as 15 x 26 x 7 and 17 x 28 x 7, also turn up in some hubs. Because wheelsets vary, confirm against your actual bearing.
Loose ball bearings (cup and cone)
Older or lower-cost hubs, and some high-end rebuildable ones, use loose steel balls rather than cartridge bearings. The balls come in standard diameters, in fractions of an inch, and front and rear hubs often use different sizes. Check the hub maker’s specification or measure with calipers, and replace all the balls on a side at once. Cup-and-cone hubs also need correct preload adjustment, which is a skill worth learning from the hub’s service instructions or from a mechanic.
How to find your size
- Read the bearing. After removing the wheel, look for stamped codes on the bearing face. Dust seals may hide them, and they can sometimes be lifted off gently.
- Measure. Use calipers to measure the inner diameter, outer diameter and width. Compare with the manufacturer’s data.
- Check the axle. The axle diameter usually matches the bearing’s inner diameter.
- Look up the hub. Search the hub model or contact the maker for a parts list or service manual.
- Ask a shop. If you are unsure, bring the old bearing to a bike shop.
See our bike hub diagram and parts page to find where bearings sit inside the hub.
Steel, ceramic and grades
Most bike bearings are steel. Ceramic bearings use ceramic balls and are often marketed on lower friction and durability. Real-world gains are small and depend on seals and grease too. Bearings are also given a precision grade (for example ABEC ratings). Higher grades are not necessarily better for bike hubs. Seal quality, lubrication and correct fit tend to matter more.
Replacing bearings: precautions
- Use the right tools to remove and press in bearings. Hammering on a bearing’s inner race or pressing it unevenly can damage it or the hub.
- Press on the outer race when installing into the hub shell.
- Some hubs use unusual bearings or specific tool requirements. Follow the manufacturer’s service instructions.
- If a hub shell has damaged bearing seats, it may need replacing.
When to check bearings
Check your bearings if the wheel feels rough when spun, if you feel play (side-to-side movement) at the rim, or after riding in heavy rain or dust. Bearings that fail can also affect how the wheel sits in the frame. For how the hub fits in the wheel overall, see the anatomy of a bike wheel, and see bike wheel axle types for axle sizes.