What the last two digits of a bearing number actually mean

A bearing designation is read from the right. The last two digits are the bore code, and for most of this library the bore in millimetres is that code multiplied by five: 04 is 20 mm, 05 is 25 mm, 20 is 100 mm. 492 of the 764 bearings here follow that rule, across all 9 types.

The multiplication is not a conversion factor — it is the step size. The code counts 5 mm steps upward from 04, so multiplying by five recovers the diameter the code was stepping to. That also fixes what the arithmetic can and cannot say: it describes the bore only, and says nothing about the digits in front of it.

The other 272 bearings in this database do not follow it, and the interesting part is that they fail in 4 unrelated ways. Each is a real property of a real kind of bearing, not a data gap. What follows is what each one is, in the order you are likely to meet them.

The 4 codes below 04

69 bearings here carry one of 4 codes that break the arithmetic on purpose.

The smallest bores sit at sizes where a 5 mm step is too coarse, so the codes were never given an arithmetic. 00 is not 0 mm, and 03 is not 15 mm.

The last column is what makes this worth its own section. 2 of the 4 arithmetic results land exactly on another code's real bore — 02 × 5 is 10 mm, which is the true bore of 00. So the mistake does not produce an obviously wrong number; it produces a size that exists, on a page, for a bearing you did not ask about. A rule that is right 492 times and quietly plausible here is worse than a rule that is obviously missing.

When the bore is not a multiple of five

9 bearings here carry a code written with a slash — /22, for a 22 mm bore. The code writes the millimetres directly instead of inventing one, so the slash is the signal that no arithmetic applies at all. All of them are in the needle roller ranges.

The code may not name the bore you need

94 bearings here have no bore, and their code still reads one. These are the needle bearings without an inner ring: the shaft itself is the inner race, so the part has no inside diameter of its own to measure.

RNA4900 is the pattern. Its code 00 reads 10 mm — the bore a complete bearing of that size would have, and the number every table of that series is indexed by. This one runs on a 14 mm shaft.

The gap between those two numbers is the two rows of needles the ring sits in, and it is not a constant. Across the 94 of them here it runs from 4 to 30 mm, in 10 different values. There is no offset to memorise; there is a table to read.

The clearest way to see what the code is really naming is to put the two versions side by side. NA4900 is the same bearing with an inner ring: 10 mm bore, running on a 10 mm shaft. Take the ring out and you have RNA4900, running on a 14 mm shaft. Same outside diameter, same width, different shaft — because the ring that used to be there is now the shaft.

That is why every bearing page here prints the code's reading and the shaft size as two separate numbers, and why the shaft size is the one to design around.

Drawn cups: neither number is a bore

The 100 drawn cups in this database are a different case again. HK1210 reads as three parts: a two-letter prefix, then 12, then 10. Neither number is a bore. The first is Fw, the diameter the needle cage runs on — which is the shaft size — and the second is C, the width.

The outside diameter is not in the designation at all. Everything sharing that same Fw of 12 mm — the same two digits in the same position — still spans 16 to 18 mm outside:

Read those two digits and you have the shaft size and the width. The outside diameter comes from the table: it moves with the width — HK1210 is 16 mm at 10 mm wide, HK1212 is 18 mm at 12 mm — so neither digit leads you to it.

What the front of the number does

Everything so far has been about the last two digits. The rest of the designation is a different job, and it is not a size either.

40 mm is the bore this database lists most often: 26 bearings share it. Their outside diameters run across 8 values from 50 to 110 mm, and their widths from 6 to 40 mm.

Nothing in the bore code separates any of those rows. The digits in front of it select the diameter series — how much outside diameter is built around a given bore — and the width series; the leading character selects the type. That is the whole answer to "which 40 mm bearing do I need": the last two digits got you to the row, and the rest of the designation is what decides whether that row suits the load, the speed and the space.

A note on suffixes

Real designations carry more than a basic code: 6205-2RS, 6205 C3, 6205/P6. Those suffixes are separate decisions that this database does not encode.

• Seals and shields (-2RS, -ZZ) change friction, speed limits and what the bearing keeps out. None of that is here.
• Internal clearance (C2, CN, C3, C4, C5) is a range on the diametral play. 4 of the 9 types carry a clearance table here, 5 groups each; the other 5 carry none. The table gives the range for a code — it is not a decoder for the suffix on someone else's label.
• Tolerance class (P6, P5) sets how closely the bore and outside diameter are held. Not in this database.
• Cage type is a material and design choice behind the basic code. Not in this database.

What every bearing page here prints is the boundary dimensions — bore, outside diameter, width or height — which are the standardised values. Load ratings, limiting speeds and mass are catalogue figures that differ between makers for the same designation, so they are deliberately absent rather than averaged into something that looks authoritative.

Reading a designation you were given

1. Read the last two digits. From 04 up, multiply by five and check it against the shaft.
2. If the code is 00 to 03, read the bore off the table above — the multiplication gives an answer that may look like a size.
3. If there is a slash, the number after it is already millimetres.
4. If the part has no inner ring, the code names the complete bearing, not your shaft. Look up the shaft size.
5. Then go back to the front of the designation for the outside diameter and width.

The bearing index lists all 764 in 9 types, and each series page prints the full dimension table for its own bore range. The standards index says which standard fixes which of the dimensions above.