How to read a bearing number, explained

Bearing designations look like part numbers but read like sentences — the same grammar across every maker, which is why a smeared 6204-2RS on a dead bearing is a complete replacement order. Read right to left.

The grammar: type · series · bore

6204 → 6 (deep-groove ball) · 2 (light series) · 04 (bore 4×5 = 20 mm)

Bore code (last two digits): 04 and up multiply by five. The four small ones are memorized exceptions — 00 = 10, 01 = 12, 02 = 15, 03 = 17 mm. Series (next digit) sets the cross-section for that bore: 9 ultralight, 0/1 extra-light, 2 light, 3 medium, 4 heavy — same bore, fatter rings, more capacity. Type (leading digit): 6 deep-groove ball, 7 angular contact, 5 thrust, 2 spherical roller, 3 taper roller (which also uses its own ISO 355/inch systems), N-prefixes cylindrical roller. A 6304 is therefore the same 20 mm bore as a 6204 with a heavier ring section.

Suffixes: closures, clearance, cage

After the size: Z/ZZ metal shields (non-contact, high speed), RS/2RS rubber contact seals (better exclusion, more drag), C2–C4 internal radial clearance (C3 looser than standard — the default choice where the inner ring runs hot and grows, like motor shafts). Cage and grease codes vary by maker; size and closure grammar do not.

From the number to a life estimate

The designation pins the catalog row, and the row's dynamic rating C feeds the life math:

L10 = (C/P)³ million rev (ball) · hours = L10 × 10⁶ / (60·n)

Say the catalog lists C = 12.8 kN for your 6204 and the equivalent load works out to 2 kN: (6.4)³ ≈ 262 million revolutions — about 2,400 hours at 1,800 RPM, per the bearing L10 calculator (rollers use the 10/3 exponent). The cubic is the design lever: drop the load 20% and life nearly doubles. The L10 guide covers what the "10" actually promises — and the shaft the bearing rides on still answers to the shaft diameter and fit checks.

Common mistakes

• Multiplying the small codes by five. 6201 is a 12 mm bore, not 5 — the 00–03 exceptions predate the ×5 rule and never left.
• Treating C3 as "premium". It is looser, not better — in a cold, precisely-fitted application C3 clearance is rattle; in a hot motor it is survival. Match it to the thermal case.
• Replacing seals with shields (or vice versa) "same number". 6204-ZZ and 6204-2RS interchange dimensionally but not functionally — washdown duty eats shielded bearings.
• Reading C from a different maker's catalog than the bearing. Ratings for the same designation differ a few percent between brands and editions; the life math deserves the rating of the bearing actually fitted.

Frequently asked questions

What does the bearing number 6204 mean?

Read it right to left: 04 is the bore code (codes 04 and up multiply by 5 → 20 mm bore), 2 is the dimension series (light), 6 is the type (single-row deep-groove ball). So 6204 = a 20 mm bore, light-series deep-groove ball bearing — the most common bearing designation on earth.

How do bearing bore codes work?

Codes 04 and larger multiply by 5: 04 = 20 mm, 05 = 25, 06 = 30, 08 = 40, 12 = 60. The first four are exceptions to memorize: 00 = 10 mm, 01 = 12, 02 = 15, 03 = 17. Below that, the bore appears directly (e.g. 625 → 5 mm). Same code system across makers — a 6204 is a 6204 everywhere.

What do bearing suffixes like ZZ, 2RS and C3 mean?

Closures and fits: Z = one metal shield, ZZ = both sides; RS = one rubber contact seal, 2RS = both. C3 is internal clearance looser than standard — for shafts that grow with heat (motors); C2 is tighter. So 6204-2RS-C3 is the 20 mm deep-groove, sealed both sides, loose clearance.

How long will a 6204 bearing last?

From the catalog dynamic rating and your load: L10 = (C/P)³ million revolutions for ball bearings. With a catalog C of 12.8 kN and a 2 kN equivalent load, (6.4)³ ≈ 262 million revs — about 2,400 hours at 1,800 RPM, the life 90% of a batch meets or beats. Halve the load and life grows eightfold.