6204 vs 6304: Same Bore, Different Capacity
Same shaft, different bearing
The 6204 and the 6304 share the same 20 mm bore — both slide onto the same shaft. What differs is everything outside the bore: the 6304 belongs to the heavier 03 dimension series, so it gets a larger outside diameter, more width, bigger balls — and a clearly higher load capacity.
| 6204 (series 02) | 6304 (series 03) | |
|---|---|---|
| Bore (d) | 20 mm | 20 mm |
| Outside diameter (D) | 47 mm | 52 mm |
| Width (B) | 14 mm | 15 mm |
| Dynamic load rating (approx.) | 13.5–14.7 kN* | 15.9–17.6 kN* |
| Static load rating (approx.) | 6.6–6.7 kN* | 7.8–7.9 kN* |
Note * Ratings vary slightly between manufacturers; the ranges above span the five catalogue brands listed on the 6204 and 6304 pages.
What the extra size buys you
- About 20–25% more dynamic capacity. That translates directly into fatigue life: under the same load, a 6304 lasts roughly twice as long as a 6204, because bearing life scales with the cube of the load ratio for ball bearings.
- About 19% more static capacity — useful against shock loads and permanent deformation at standstill.
What it costs you
- Speed. The larger balls and heavier construction run hotter at speed. Depending on brand and closure, the catalogue limiting speed of an open 6304 sits around 10–20% below an equivalent 6204.
- Space and mass. You need 5 mm more radial room and a slightly wider housing seat.
- Price. A 6304 typically costs a little more, though both are high-volume commodity sizes.
They are not drop-in replacements for each other
The shaft fit is identical, but the housing bore is not: 47 mm versus 52 mm. Swapping a 6204 for a 6304 means remachining the housing. If you need more capacity without touching the housing, the realistic options are a higher-quality 6204 (better steel, tighter grade) or fixing the root cause — alignment, lubrication or overload — rather than upsizing.
Which one should you pick?
- Space is tight or speed is high → 6204. It is the default choice for small electric motors, pumps and general machinery on a 20 mm shaft.
- Load or shock is the problem, and you have 5 mm to spare → 6304. Common in gearboxes, agricultural equipment and anything that sees abuse.
- Unsure → start from the load, not the bearing. Work out your radial and axial loads, then check the rating tables.
Next step: compare the full numbers on the 6204 and 6304 detail pages, or walk through the selection guide to size a bearing from your operating conditions.