6304-ZZ bearing: complete specifications & equivalents
The 6304-ZZ is a ISO 15 radial bearing sized 20 x 52 x 15 mm (bore x OD x width) with ZZ sealing. Verify dimensions against the official catalog before ordering.
Specification table
| Parameter | Value |
|---|---|
| Brand | SKF |
| Bore diameter d | 20 mm |
| Outer diameter D | 52 mm |
| Width B | 15 mm |
| Material | GCr15 |
| Seal / shield | ZZ |
| Internal clearance | CN / C3 available |
| Tolerance class | P0 |
| Cage | steel, stamped ribbon (J-type) |
| Basic dynamic load rating C | 16.8 kN |
| Basic static load rating C0 | 7.8 kN |
| Fatigue load limit Pu | 0.335 kN |
| Reference speed (thermal) | 30000 rpm |
| Limiting speed (mechanical) | 15000 rpm |
| Operating temperature | -30 to +120 °C |
| Mass | 150 g |
| Manufacturer designation | SKF 6304-2Z |
| Standard | ISO 15 |
| Catalog version | SKF PUB BU/P1 17000/1 EN (Oct 2018) |
| Last verified | 2026-09-25 |
Source: catalog reference (SKF PUB BU/P1 17000/1 EN (Oct 2018)) · extracted 2026-09-24
Parameters extracted from the official SKF Rolling Bearings catalogue (SKF PUB BU/P1 17000/1 EN (Oct 2018)); SKF® and SKF Explorer® are trademarks of AB SKF, used here for identification only — mpnbase is not an authorized SKF distributor. Boundary dimensions are ISO 15 interchangeable; current production values may differ, verify at skf.com.
Dimensions in inches & 6300-series range
| Bore d | 0.787 in |
|---|---|
| Outer diameter D | 2.047 in |
| Width B | 0.591 in |
| ISO bore code | 04 |
| Radial section height (D−d)/2 | 16 mm |
| Smallest bore in 6300-series | 2 mm |
| Largest outer diameter in 6300-series | 500 mm |
| Width range across 6300-series | 3–95 mm |
| Bore position in 6300-series (smallest first) | 9 of 40 |
| Bore above smallest 6300-series size | 18 mm |
Mounting, lubrication & life for 6304-ZZ
Recommended fits & mounting
- Rotating inner ring on a 20 mm shaft: k5; housing bore H7 for normal stationary-load outer rings. Reverse the logic if the outer ring rotates.
- Fit by induction heating (max 120 °C) where practical — never drive the ring home through the rolling elements.
- Keep shaft and housing fillet radii below the bearing chamfer so the 15 mm-wide ring seats square on its shoulder.
Lubrication strategy
Non-contact metal shields block coarse debris while running nearly as cool as open. No relubrication path on the standard execution; the shield cavity grease ages even though the steel is still serviceable.
Worked L10 life example (illustrative duty)
For ball bearings, L10h = (C/P)3 · 106 / (60·n). Taking an example radial equivalent load P = 3 kN at n = 1,500 r/min (a small belted motor duty — substitute the real equivalent load from the application calculation):
C = 16.8 kN → L10h ≈ 1,951 hours — a warning that this section is lightly rated for a 3 kN load; a heavier section at the same 20 mm bore is the usual fix
Choosing a section at the same 20 mm bore
| Series | Open execution | OD × width | C (kN) | Limiting speed (r/min) |
|---|---|---|---|---|
| 60xx | 6004-FAG | 42 × 12 | 10.6 | 27,500 |
| 62xx | 6204-FAG | 47 × 14 | 14.7 | 24,300 |
| 63xx | 6304-FAG | 52 × 15 | 16.9 | 19,800 |
| 64xx | 6404-FAG | 72 × 19 | 30.5 | 14,000 |
| 618xx | 61804-FAG | 32 × 7 | 4.0 | 33,500 |
| 619xx | 61904-FAG | 37 × 9 | 6.4 | 28,500 |
| 160xx | 16004 | 42 × 8 | 7.9 | 26,000 |
A larger section is not a drop-in swap — OD and width change — and same-envelope seal variants are compared on the 20×52×15 envelope page. Match meaning, not the size chart alone: see what the cross-reference levels mean.
Cross references & equivalents
| Equivalent model | Brand | Match level | Note |
|---|---|---|---|
| 6304-2RS-FAG | FAG | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 20×52×15 mm envelope per ISO 15; seal variant differs (ZZ vs 2RS) — confirm sealing suits the duty. |
| 6304-FAG | FAG | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 20×52×15 mm envelope per ISO 15; seal variant differs (ZZ vs OPEN) — confirm sealing suits the duty. |
| 6304-ZZ-FAG | FAG | full Dimensions and internal design equivalent — standard interchange. | Identical ISO 15 envelope, seal and ratings band — standard interchange; re-verify precision class and clearance per supplier. |
| 6304 | ISO 15 | verify Commonly quoted cross reference; confirm against the target catalog before ordering. | ISO 15 boundary-dimension envelope; seal suffix convention varies by brand (2RS / DDU / LLU / 2RSR) — verify seal design and temperature range before substituting. |
| 6304-2RS-NSK | NSK | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 20×52×15 mm envelope per ISO 15; seal variant differs (ZZ vs 2RS) — confirm sealing suits the duty. |
| 6304-NSK | NSK | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 20×52×15 mm envelope per ISO 15; seal variant differs (ZZ vs OPEN) — confirm sealing suits the duty. |
| 6304-RSR-NSK | NSK | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 20×52×15 mm envelope per ISO 15; seal variant differs (ZZ vs RSR) — confirm sealing suits the duty. |
| 6304-ZZ-NSK | NSK | full Dimensions and internal design equivalent — standard interchange. | Identical ISO 15 envelope, seal and ratings band — standard interchange; re-verify precision class and clearance per supplier. |
| 6304-2RS-NTN | NTN | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 20×52×15 mm envelope per ISO 15; seal variant differs (ZZ vs 2RS) — confirm sealing suits the duty. |
| 6304-NTN | NTN | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 20×52×15 mm envelope per ISO 15; seal variant differs (ZZ vs OPEN) — confirm sealing suits the duty. |
| 6304-RSR-NTN | NTN | partial Main dimensions match; tolerances, seals or internal clearance differ. | Same 20×52×15 mm envelope per ISO 15; seal variant differs (ZZ vs RSR) — confirm sealing suits the duty. |
| 6304-ZZ-NTN | NTN | full Dimensions and internal design equivalent — standard interchange. | Identical ISO 15 envelope, seal and ratings band — standard interchange; re-verify precision class and clearance per supplier. |
| 7304 | SKF | verify Commonly quoted cross reference; confirm against the target catalog before ordering. | — |
| NU304 | SKF | verify Commonly quoted cross reference; confirm against the target catalog before ordering. | — |
| W6304-2RS | SKF | partial Main dimensions match; tolerances, seals or internal clearance differ. | SKF stainless execution W 6304-2RS1 shares the 20×52×15 mm envelope but rates differently (16.8/7.8 kN vs 13.8/7.8 kN) — not a load-rating equivalent. |
| W6304-ZZ | SKF | partial Main dimensions match; tolerances, seals or internal clearance differ. | SKF stainless execution W 6304-2Z shares the 20×52×15 mm envelope but rates differently (16.8/7.8 kN vs 13.8/7.8 kN) — not a load-rating equivalent. |
| W6304 | SKF | partial Main dimensions match; tolerances, seals or internal clearance differ. | SKF stainless execution W 6304 shares the 20×52×15 mm envelope but rates differently (16.8/7.8 kN vs 13.8/7.8 kN) — not a load-rating equivalent. |
Interchange levels: full standard interchange · partial verify parameters · verify manual confirmation required. Links resolve to the equivalent model's specification page where available.
How the 6304-ZZ model number is decoded
In the 6300-series (diameter series 3 of ISO 15) the last two digits encode the bore: 6304 carries bore code 04 = 20 mm. Outside diameter 52 mm and width 15 mm come from the ISO 15 boundary-dimensions general plan. Suffix -ZZ denotes two non-contact metallic shields; they retain grease and block coarse debris but are not liquid-tight. For normal-load mounting with a rotating inner ring on a 20 mm shaft, typical catalogue practice is a k5 shaft seat with an H7 housing bore — verify against the actual load direction and magnitude. SKF's own suffix is 2Z (this page's industry-neutral spelling is -ZZ); catalogue number 6304-2Z.
Typical equipment & failure modes
Common applications
- conveyors and drives
- agricultural equipment
- fans and blowers
- electric motors
Known failure patterns
- fine-particle abrasion when non-contact shields let dust past in dirty duty
- grease aging in the shield-for-life cavity with no relubrication path
- false brinelling from vibration while the shaft is stationary
- fretting corrosion on a loose shaft seat
Selection pitfalls — read before you substitute
- A 20 mm shaft for 6304 should be finished to Ra ≤ 1.6 µm with square seating faces; at this bore a scored or oval journal shows up directly as running noise and early raceway damage.
- The 63xx medium section at 20 mm bore gains dynamic rating through larger rolling elements on a 52 mm OD; the penalty is mass and a lower limiting speed than the 6204 — do not specify it for speed-critical shafts.
- ZZ shields are non-contact: 6304-ZZ blocks chips and coarse dust but is not waterproof — do not install it expecting a pressure-wash seal, step up to 6304-2RS for that.
- Do not wash 6304-ZZ in solvent before mounting: the grease inside the shielded cavity is the service fill, and solvent creeping past the shield gap strips it out.
- A 20 mm bore listing showing the right d/D/B but no standard or precision-class reference may be a loose-tolerance import; measure bore/OD spread before fitting it to a machined shaft.
- Deep-groove ball bearings tolerate only minutes of misalignment; if the 52 mm housing cannot be aligned, switch to a self-aligning type rather than shimming 6304-ZZ.
- SKF spells the shielded version 6304-2Z (one shield: 6304-Z), not "-ZZ": the suffix language differs by brand even though the ISO envelope is identical, so quote the brand designation when ordering.
- The single-shield execution 6304-Z is a distinct order number in catalogue 17000: it suits open gears or oil-bath arrangements where the unshielded face faces the lubricant, and it is not interchangeable with the double-shield 6304-2Z for high-contamination positions.
- The 16.8/7.8 kN ratings here are SKF Explorer-class figures from catalogue PUB BU/P1 17000/1 EN (Oct 2018); SKF can revise internal geometry and ratings without changing the designation, so treat the catalogue version and date as part of the specification and re-verify for critical drives.
Frequently asked questions
What are the dimensions of a 6304-ZZ bearing?
A 6304-ZZ deep-groove ball bearing has a 20 mm bore, 52 mm outer diameter and 15 mm width (0.787 × 2.047 × 0.591 in), per ISO 15 boundary dimensions. It is fitted with two non-contact metallic shields.
Are 6304 bearings from different brands interchangeable?
For boundary dimensions, yes: every brand's 6304 uses the same ISO 15 envelope (20×52×15 mm). Load ratings, limiting speeds, clearance, precision class and sealing design can differ, so verify those in the supplier catalogue when substituting. Seal suffixes also differ by brand (2RS, DDU, LLU, 2RSL and similar).
What internal clearance does 6304-ZZ have?
Unless a clearance suffix says otherwise, 6304-ZZ is manufactured to normal (CN) radial internal clearance. Tight fits and hot shafts consume clearance in service — for those conditions order the C3 execution of 6304 and verify the residual clearance after mounting.
How should 6304-ZZ be mounted?
Apply the mounting force to the ring being fitted: on a 20 mm shaft seat the press load must pass through the inner-ring face only. A fitting sleeve and an arbor press are normally sufficient at this size. Never strike the outer ring, the cage or the seals.
What is the SKF designation and catalogue data for 6304-ZZ?
In the SKF Rolling Bearings catalogue this size is listed as 6304-2Z — two non-contact sheet-metal shields, 20×52×15 mm per ISO 15. Quoted values: C 16.8 kN, C0 7.8 kN, Pu 0.335 kN, limiting speed 15000 r/min, mass 150 g. Brand suffixes (2Z, 2RSH, 2RSL) differ from the industry-neutral spelling 6304-ZZ.
Reviewed & verified by the MPN Base engineering desk against source catalog SKF PUB BU/P1 17000/1 EN (Oct 2018) on 2026-09-25. Read our editorial & correction policy.