Verified by Rachel Mayfield, Supply Chain Analyst - August 2026
The 6904 is a single-row deep groove ball bearing measuring 20x37x9 mm (bore x outside diameter x width). Those boundary dimensions come from ISO 15, so a 6904 from SKF, NSK, NTN, EZO, NMB or any budget brand is dimensionally interchangeable - the differences are in materials, precision grade, internal clearance and seal design, not size.
6904 bearing dimensions
| Property | Value | Standard |
|---|
| Bore diameter (d) | 20 mm | ISO 15 |
| Outside diameter (D) | 37 mm | ISO 15 |
| Width (B) | 9 mm | ISO 15 |
| Bearing type | Single-row deep groove ball bearing | ISO 15 |
| Dimension series | 6900 thin section (ISO dimension series 19) | ISO 15 |
Always measure a worn bearing before ordering if the number is unreadable: bore with calipers on the inner ring, outside diameter across the outer ring, and width across either ring face. 20x37x9 mm identifies the 6904 uniquely among common metric sizes.
6904 or 61904? Same bearing
The formal ISO designation for this size is 61904: dimension series 19 on a 20 mm bore. Most catalogues, bike shops and marketplaces shorten it to 6904. SKF and Schaeffler list it as 61904; NSK, NTN and most distributors sell it as 6904. It is the same bearing - order by whichever number the supplier uses and check the 20x37x9 mm dimensions match.
What the suffixes mean (6904ZZ, 6904-2RS, 6904-C3)
| Suffix | Meaning | When to choose it |
|---|
| ZZ / 2Z | Metal shield both sides | Non-contact, low friction, keeps debris out. Not washdown-proof. |
| 2RS / 2RS1 / DDU / LLU | Rubber seal both sides | Contact seals, best dirt and moisture exclusion, slightly more friction. |
| Z / RS | Shield or seal one side only | Open on the other side - used where relubrication is needed. |
| Open (no suffix) | No seals or shields | For housings with their own sealing and external lubrication. |
| C3 | Extra internal clearance | For motors and anywhere the inner ring runs hotter than the outer. |
| 2RS-C3 | Sealed + C3 clearance | The standard electric-motor spec. |
Brand equivalents for the 6904
Every major manufacturer makes the 6904. The seal and shield codes differ by brand but the bearings interchange directly:
| Brand | Shielded version | Sealed version |
|---|
| SKF | 6904-2Z | 6904-2RSH |
| FAG (Schaeffler) | 6904-2ZR | 6904-2RSR |
| NSK | 6904ZZ | 6904DDU |
| NTN | 6904ZZ | 6904LLU |
| Koyo (JTEKT) | 6904ZZ | 6904-2RS |
Load ratings: check the maker's catalogue
Dynamic (C) and static (C0) load ratings for the 6904 differ between manufacturers and between standard and premium ranges, so MLC does not publish a single figure. Look the value up in the manufacturer's own catalogue for the exact variant you are buying - SKF, Schaeffler medias, NSK and NTN all publish free online catalogues with per-variant ratings, limiting speeds and grease specifications.
Where the 6904 is used
The 6900 thin section (ISO dimension series 19) family is a thin section one step heavier than the 6800 series - the usual pick for bicycle hubs and hub motors. Typical 6904 applications include e-bike hub motors, scooters and compact gearboxes.
Fitting basics
For a rotating shaft (the normal case), fit the inner ring with a light interference fit and the outer ring with a close clearance fit (H7 housing for most machinery). Press only on the ring you are fitting - pressing the outer ring through the balls to seat the inner ring is the fastest way to brinell a new bearing. For sealed 2RS variants, no relubrication is needed or possible; the factory grease fill is for life.