Verified by Rachel Mayfield, Supply Chain Analyst - July 2026
Photo by Marcelo Avila on Pexels
The 6304 is a single-row deep groove ball bearing with a 20 mm bore, 52 mm outside diameter and 15 mm width (20x52x15 mm). Those boundary dimensions are set by ISO 15, so a 6304 from SKF, FAG, NSK, NTN, Koyo or any budget brand is dimensionally interchangeable - the differences are in materials, precision grade, internal clearance and seal design, not size.
6304 bearing dimensions
| Property | Value | Standard |
|---|
| Bore diameter (d) | 20 mm | ISO 15 |
| Outside diameter (D) | 52 mm | ISO 15 |
| Width (B) | 15 mm | ISO 15 |
| Bearing type | Single-row deep groove ball bearing | ISO 15 |
| Dimension series | 6300 (heavy) | 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. 20x52x15 mm identifies the 6304 uniquely among common metric sizes.
What the suffixes mean (6304ZZ, 6304-2RS, 6304-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 6304
Every major manufacturer makes the 6304. The seal and shield codes differ by brand but the bearings interchange directly:
| Brand | Shielded version | Sealed version |
|---|
| SKF | 6304-2Z | 6304-2RSH |
| FAG (Schaeffler) | 6304-2ZR | 6304-2RSR |
| NSK | 6304ZZ | 6304DDU |
| NTN | 6304ZZ | 6304LLU |
| Koyo (JTEKT) | 6304ZZ | 6304-2RS |
Load ratings: check the maker's catalogue
Dynamic (C) and static (C0) load ratings for the 6304 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 6304 is used
The 6300 (heavy) series is thickest section of the three, highest load capacity. Typical 6304 applications include electric motors, gearboxes, pumps, fans, conveyor rollers, power tools and general machinery - anywhere a rotating shaft of 20 mm needs simple radial support with modest axial capability.
Fitting basics
For a rotating shaft (the normal case), fit the inner ring with a light interference (k5/k6 shaft tolerance for most machinery) and the outer ring with a close clearance fit (H7 housing). 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.