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Standard bicycle bearing ball sizes

Bicycles usually use ball bearings. Modern cartridge bearings come as a closed unit – with balls and races packed in a single unit. However, many bicycles still use older, cup and cone system, where balls can (and should) be replaced with new ones when servicing bearings. This is the case with most Shimano hubs (wheels) – they still use the cup and cone system. This post lists standard ball sizes and gives guides on how to choose a good quality ones, without the need to buy special, branded “bicycle bearing balls”.

Post explaining bicycle bearing types and construction: Types and designs of bicycle bearings.
Headset (fork bearings) standards explained: Bicycle headset bearings standards – SHIS.
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1. Quality (manufacturing precision) grades of bearing balls

Grade defines tolerance (i.e. acceptable deviation) from a perfect sphere of a given diameter. Grades are noted in numbers, ranging from 3 to 2000. They are written in the format: GXXXX, where XXXX is a number ranging from 3 to 2000. The number represents how many 1/1000 inches is maximal deviation from nominal dimension.

The lower the number after G mark, the closer a ball is to a perfect sphere of a given dimension. Ball imperfections come in various terms:

  • surface roughness
  • waviness
  • egg shapeness

For example, a ball of a grade G1000, with a nominal diameter of 6.35 mm (1/4″), can have a diameter varying from that size for over one tenth of a milimetre. On the other hand, a G10 ball of the same nominal size will have a diameter more closely matching the nominal, with just a ±0.0013 mm maximal deviation. Similar goes for other types of deviations (surface roughness etc.). The lower the grade number, the closer a ball is to a perfect shape.

Since cups and cones in bicycle bearings have some elasticity and imperfections as well, balls of grade G25 are more than precisely enough machined (round and smooth). Going for higher grade balls (e.g. G10, G5 etc.) will not bring any performance, or durability benefits, just the balls will cost (needlessly) more.

2. Bearing ball hardness

Bearing ball hardness is usually measured and expressed with Rockwell HRC method. Values for hardened steel are usually in the range of 40 to 70. If a bearing ball is too soft, it will wear rather quickly, then start damaging the cup and cone bearing because of increased play and imperfect alignment. On the other hand, if a bearing ball is too hard, instead of it getting worn, it will start wearing (softer) cups and cones of the bearing – that are more expensive and complicated to replace. That is why it is important for the balls to be of optimal hardness. Optimal hardness for bicycle ball bearings is between 55 and 65 HRC.

3. Standard dimensions of bicycle bearing balls

Here’s a list of standard, most commonly used dimensions of bearing balls. There are other, exotic sizes, but they are extremely rare. To be on the safe side, when servicing bearings (and replacing balls then, which is always recommended), measure old balls. Best measured with a (Vernier) caliper.

Measuring, in this case a rear hub bearing ball, with digital calipers, showind dimension in inches
Measuring, in this case a rear hub bearing ball, with digital calipers, showind dimension in inches
  • 3.969 mm (5/32″) – steerer (fork) bearings and many pedals.
  • 4.762 mm (3/16″) – front wheel hubs (2 times 10 balls – i.e. 10 balls at each side most often).
  • 6.35 mm (1/4″) – rear wheel hubs (usually 2 times 9 balls) and cup and cone bottom brackets (2 times 11).
  • 5.556 mm (7/32″) Campagnolo Record front hubs and some exotic hubs of other manufacturers (2 times 9 balls).
  • 2.381 mm (3/32″) – most Shimano pedals bearing balls.

4. Conclusion

It is good to replace bearing balls at each bearing service. They can be purchased rather cheaply (by 1000) from most hardware stores. Just make sure they are of proper grade (G25 or better) and hardness (HRC 55 to 65).

There is little point going over G25 grade, and absolutely no point going better than G10 (even that is an “overkill” for this application). Similarly, using balls softer than 55 HRC, or harder than 65 HRC can only decrease bearings lifespan.

Related post – Bicycle bearing grease:

What is the best bicycle bearing grease? Which type of grease to choose for bicycle bearings?
What is the best bicycle bearing grease? Which type of grease to choose for bicycle bearings?

Links for on-line ball bearing shopping from Amazon. As an Amazon Associate I earn from qualifying purchases.

21 thoughts on “Standard bicycle bearing ball sizes”

  1. Thank you. Very helpful to know. Of course, this info might be found on internet somewhere, but i appreciate that you put such info together for readers’ convenience.

  2. Having worked in a ball bearing plant for over 20 years. A grade 25 is round within 25 millionths of an inch, and within a given lot the ball to ball diameter variation is within 50 millionths of an inch. Likewise, a grade 10 is is round within 10 millionths of an inch and ball to ball diameter variation within a lot is 20 millionths. The convention is twice the roundness typically.

    • I don’t know.
      If Google doesn’t help, suppose the only way is to measure the existing ones – if they are at hand.
      Maybe someone who knows will chime in.

  3. surface roughness
    egg shapeness
    thought I learned a new word today…wawiness

  4. This is very helpful and you have solved my problem of what size my bearings are so I can buy replacements.
    Thank you. Stuart

  5. This is a good piece of information. Thank you for sharing. I have French pedals from the 1940s, with ball bearings that I measure as 3.15mm. Everything else on that bike is non-standard, (thread pitch, bottom bracket, stem diameter, post diameter,….). Maybe the pedal bearings were uncommon too. I am still searching for new ones of the correct size.

    • 3.15mm as measured is likely just to be standard 1/8″ balls, I would have thought…

  6. I recently purchased a 26inch Beinaiqi cavalier fat bike, my question is, can I put cartridge bearings in my rear hub or should I stay with the cup & cone system ?

    • I don’t know how one would put cartridge bearings into a hub with cup and cone bearings (or vice-versa for that matter).
      The usual way of achieving that is getting a different hub.

      But I wouldn’t sweat it too much – cup and cone bearings are relatively easy to service and can last a long time if serviced regularly (generally: once a year, or each 5000 km, whichever comes first).

  7. in australia alot of people just throw old bikes into the street when they cant be bothered to fix them,i totaly strip older bikes for parts and sometimes i even find a nice spare bike to fix up,i re-use bearings,cassettes handle bars you name it,i even find new tubes and tires that are thrown away.

  8. Great explanation, thank you!
    This article just saved me a lot of time, I overhauled the wheels of 3 bikes at the same time last week , and today
    I found a ball stuck to my shoes, ~6.3mm in diameter. Where does it belong? It’s not from my old Peugeot, they use thin hubs and smaller balls. Not from the front of Raleigh, that should be 3/16″ (~4.8mm), so it should be from the rear of Raleigh! Just one wheel to open and insert the wheel, a 10 minutes job! In my defense, the bearing on the cassette side looks smaller, at the time it seemed logic to have 8 balls instead of 9.

  9. Thanks for the advice on loose bearings, the bearings in my Shimano hubs are held in a cage, can I just put new bearings in the old cage? If I can source non Shimano bearings in a cage is it enough to just match the dimensions of the cage or do they come with different profiles to fit into the hub races?

    • Hi Andy,

      I suppose you could use the old cages with new balls.
      However, I prefer tossing the cages out and putting more loose bearing balls (more will fit once the cage is out).

      An exception to that are hubs that have a huge amount of bearing balls that are too fiddly to fit one by one.


  10. cages are only made to make it easier to install bearings,they serve no other purpose,bearings run alot better without cages,those cages allways cause problems,fine bearings in cages around steering stem i only use as those bearings are just to fine to install whithout a cage.when using loose bearings leave a space,you fill the whole space with bearings then you remove one or two bearings so there is a space,if you dont leave a space between those bearings they will grind away and not run smoothly

    • Hi Lukasz,

      Thank you very much for the correction. If you notice any other mistakes, don’t hesitate to not them. It’s appreciated. 🙂


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