How to measure a bearing and find its number
No readable number on the old bearing? Measure the bore, outside diameter and width with a vernier caliper, then match them to the size chart. Here is how.
Markings wear off, rust over or get ground away, and some bearings arrive without a box. The good news is that almost every standard bearing can be identified from three measurements: the bore, the outside diameter and the width. Here is how to take them and turn them into a part number.
What you need
- A vernier or digital caliper. A ruler is not accurate enough, because neighbouring sizes often differ by only a millimetre.
- A clean cloth to wipe off grease and dirt, so you measure metal and not grime.
- The bearing out of the machine. If you cannot remove it yet, measure the shaft and the housing bore instead.
Step 1: measure the bore (d)
Use the inside jaws of the caliper across the hole in the inner ring. Take two or three readings at different angles and use the largest. If the bore is badly worn, measure the shaft it sat on instead: the shaft size is the bore size.
Step 2: measure the outside diameter (D)
Use the outside jaws across the outer ring, again in two or three directions. A reading that varies by more than a few tenths of a millimetre usually means the ring is damaged, so go with the most common value.
Step 3: measure the width (B)
Measure across the flat faces of the outer ring, not across protruding shields or seals. For a taper roller bearing, measure the overall width (T) with the cone and cup assembled.
Step 4: match the three numbers
Look up all three measurements together in our bearing size chart. For example, 20 × 47 × 14 mm is a 6204, and 17 × 40 × 12 mm is a 6203. Remember that one bore size appears in several series, from thinnest to heaviest: 68, 69, 60, 62 and 63. Only the outside diameter and width tell them apart, which is why you need all three numbers.
Then check the seals and clearance
Look at the sides of the old bearing. Metal shields mean ZZ, rubber seals mean 2RS, and no cover at all means an open bearing. Internal clearance such as C3 cannot be measured with a caliper. If the bearing came from an electric motor, it was most likely C3, but tell us the machine and we will confirm.
Readings that look like inches
If your readings come out at values such as 19.05, 22.225 or 47.625 mm, the bearing is probably an imperial (inch) size: those are 3/4, 7/8 and 1 7/8 inch. Send us the millimetre readings anyway and we will convert and match them.
Not in the chart?
Some bearings are non-standard or come from imported machines. Read what to send us for an odd-size bearing, or WhatsApp us your three measurements and a photo. We have been identifying bearings this way at our Kalbadevi counter since 1974.