Miniature ball bearing · 68x thin miniature series
684 bearing
4 × 9 × 4 mm
The 684 is a miniature ball bearing with a 4 mm bore, 9 mm outside diameter and 4 mm width. Widths shown are shielded/sealed widths (ZZ, 2RS); open versions are narrower.
- Bore d
- 4 mm 0.1575 in
- Outside diameter D
- 9 mm 0.3543 in
- Width B
- 4 mm 0.1575 in
- Dimension series
- 18
- Load directions
- Radial and light axial in both directions
Designation explained
What 684 means
Standard bearing numbers encode the type, the dimension series and the bore. Anything after the base number — seals, clearance, cage — is a suffix.
| Part | Meaning |
|---|---|
6 | Deep groove ball bearing |
8 | Dimension series 18 (thin section) |
4 | Single-digit bore code → 4 mm bore (for bores under 10 mm the code is the bore in mm) |
Variants
Common 684 variants
Suffixes change the bearing without changing its boundary dimensions. The exact letters differ between manufacturers — always order by the full designation from the maker you choose.
| Suffix | Example | Meaning |
|---|---|---|
ZZ / 2Z | 684-ZZ | Metal shields both sides — the usual version for 3D printers, fans and skateboards |
2RS | 684-2RS | Rubber seals both sides; better against dust and water, slightly more friction |
F (prefix) | F684-ZZ | Flanged outer ring for locating in a thin plate, e.g. F688ZZ |
ABEC / P5 | 684-ZZ (ABEC-7) | Precision grade claims; ABEC ratings describe tolerances, not quality or speed by themselves |
S (prefix) / -C | S684-2RS | Stainless steel (S608) or ceramic balls (hybrid) — naming varies by seller |
684-2RS (sealed)
- Rubber contact seals on both sides
- Best protection against dust, water and grease loss
- Slightly more friction and a lower speed limit
- Choose for dirty, wet or outdoor use
684-ZZ / 2Z (shielded)
- Metal shields on both sides, small gap to the inner ring
- Keeps out coarse dirt; not water-tight
- Lower friction, higher speeds
- Choose for clean, fast-running applications
Same bore
Other 4 mm bore bearings
Every standard bearing in this reference with a 4 mm bore, from light to heavy sections.
Where to buy
Find 684 at sellers
Search results on independent sellers. We are not affiliated with them, do not earn from these links and do not see their prices or stock. Results mix brands and variants — check the full designation before you order.
About these dimensions. Boundary dimensions follow ISO 15 (radial bearings — boundary dimensions, general plan), dimension series 18, and are the same for every manufacturer of a standard 684. Chamfers, load ratings, speed limits and weight depend on the manufacturer and design and are not shown — use the maker's datasheet for engineering decisions. Last checked 9 October 2026. Spotted an error? Tell us.
Learn more
What size is a 684 bearing?
A 684 is a miniature ball bearing with a 4 mm bore, 9 mm outside diameter and 4 mm width (4 × 9 × 4 mm). These boundary dimensions follow the ISO dimension plan, so they are the same across manufacturers.
Is 684-ZZ the same size as 684-2RS?
Yes, shielded (ZZ) and sealed (2RS) versions share the same bore and outside diameter; in the 68x/69x series the open version can be narrower than the 4 mm shown. ZZ runs with slightly less friction; 2RS keeps out dust and water better.
Can I replace 684 with another bearing of the same size?
Other bearing types can share the same 4 × 9 × 4 mm envelope, but a different type has different load directions, speed limits and mounting needs. Same dimensions alone do not make two bearings interchangeable.
Where are the load ratings and speed limits?
They depend on the manufacturer and internal design, so we do not list a single value. Check the datasheet for the exact full designation you intend to use.
Need 684 data for a whole catalog?
We can help with that.
Normalized specifications, variants and supplier listings for every bearing you sell, with the source behind each value.