Identify an unmarked or worn bearing
A bearing has failed, and the number that was stamped on the ring is rusted, worn or was never legible. You still need to order the right replacement. The good news: because bearing dimensions are standardised, you can almost always work back to the designation from three measurements and a look at the bearing. Here is how.
Step 1 — measure the three boundary dimensions
With a caliper (vernier or digital), measure in millimetres:
- Bore (d) — the inner diameter, where the shaft goes.
- Outer diameter (D) — the outside of the outer ring.
- Width (B) — the axial thickness. For tapered and thrust bearings this is the overall height/width; for insert (UC) bearings it is the wide inner-ring width.
Measure a worn part carefully — take the largest reading across the OD and the smallest across the bore, and round to the nearest standard value. Standard bores are 10, 12, 15, 17, then multiples of 5 (20, 25, 30…). If your bore reads 19.8 mm, it is a 20 mm bore.
Step 2 — identify the type
Look at how the bearing is built:
| What you see | Likely type |
|---|---|
| A ring of balls, single row, symmetric | Deep groove ball (6-) |
| Balls, but one shoulder cut away / runs at an angle | Angular contact (7-) |
| Two rows of balls, curved outer race, self-aligning | Self-aligning ball (12/13/22/23) |
| Straight cylindrical rollers | Cylindrical roller (NU/N…) |
| Cone-shaped rollers, separable cup + cone | Tapered roller (3-) |
| Two rows of barrel rollers, curved outer race | Spherical roller (2xxxx) |
| Thin long rollers (needles) | Needle roller (NA/RNA) |
| Two flat washers with balls between (axial) | Thrust ball (5-) |
| Spherical outer surface, grub screws in the inner ring | Insert / mounted (UC) |
Step 3 — work back to the designation
Now combine type + bore. The bore code is the last two digits of the number: 00 = 10, 01 = 12, 02 = 15, 03 = 17, and 04 and up = digits × 5. So a deep groove ball bearing with a 20 mm bore is a 62__ / 60__ / 63__ ending in 04.
Which series (60/62/63…) it is comes from the outer diameter. For a 20 mm bore deep groove bearing:
- OD 42 mm → 6004 (light)
- OD 47 mm → 6204 (medium — the most common)
- OD 52 mm → 6304 (heavy)
- OD 32 mm → 6804 (thin)
- OD 37 mm → 6904
So "deep groove ball, 20 × 47 × 14" is a 6204. This is exactly what a dimension cross-reference lets you do in reverse: search the three numbers and read off the designation and every brand that makes it.
Step 4 — recover the suffixes you can
The base number gets you a bearing that fits. Try to recover the details that decide whether it works (see what interchangeable really means):
- Seals/shields — was it open, metal-shielded (ZZ) or rubber-sealed (2RS)? You can usually see this on the failed part.
- Clearance — harder to recover. If the machine ran hot or had a heavy press fit, it may well have been C3. When in doubt, check the equipment's parts list or match what the OEM specified.
- Cage and precision — usually standard for general machinery unless it is a spindle or high-speed application.
A few practical cautions
- Measure the shaft and housing too — if the bearing spun on the shaft, the seats may be worn and need attention before the new bearing goes in.
- Watch for inch bearings (common in older or North American machinery and in tapered wheel bearings). A bore of 19.05 mm is 3/4 inch, not a mis-measured 20 mm.
- If two candidates fit the dimensions but differ in type (e.g. NU vs N, or a 6206 vs 7206), go back to how it was built and how the shaft is located.
General reference only. Confirm the identified designation and its suffixes against a manufacturer catalogue, and inspect the shaft and housing before fitting a replacement.