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 systems, 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 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.
Cup and cone bicycle hub overhaul tools and procedure: Bicycle hub overhaul.
If you have any questions (or additions and corrections), please use the BikeGremlin forum:
www.bikegremlin.net
Table Of Contents (T.O.C.):
- Quality (manufacturing precision) grades of bearing balls
- Bearing ball hardness
- Standard dimensions of bicycle bearing balls
- Conclusion
- Bearing ball dimension standards
5.1. ABMA Std-10
5.2. ISO 3290
5.3. DIN 5401
5.4. Ball size, grade and material table
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 millionths of an inch is the maximal deviation from the nominal dimension (with G1000 having a 1/1000″ deviation in sphericity, and G10 having a 1/100000″ deviation).
The lower the number after the 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 shapedness (as a deviation from perfect sphericity)
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 half of one-tenth of a millimetre. On the other hand, a G25 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 using the 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, and 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. The 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) calliper.
- 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 – this includes all the Shimano dynamo hubs, except models DH-UR705, and DH-UR700).
- 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 in going over the 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.
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5. Bearing ball dimension standards
Here are some charts showing bearing ball grades and dimension standards according to ABMA Std-10, ISO 3290, and DIN 5401:
5.1. ABMA Std-10
| Grade | Sphericity | Diameter tolerance per lot | Diameter tolerance per shipment | Surface finish | |||
|---|---|---|---|---|---|---|---|
| G | inch | mm | inch | mm | inch | mm | Arithmetic average |
| 10 | 0.00001 | 0.0002 | 0.00002 | 0.0005 | ±0.0001 | ±0.00254 | 1 |
| 15 | 0.00001 | 0.0004 | 0.00003 | 0.0007 | ±0.0001 | ±0.00254 | 1 |
| 25 | 0.000025 | 0.0006 | 0.00005 | 0.0013 | ±0.0001 | ±0.00254 | 2 |
| 50 | 0.00005 | 0.0013 | 0.0001 | 0.0025 | ±0.0002 | ±0.0050 | 3 |
| 100 | 0.0001 | 0.0025 | 0.0002 | 0.0050 | ±0.0005 | ±0.0127 | 5 |
| 200 | 0.0002 | 0.0051 | 0.0004 | 0.0102 | ±0.0010 | ±0.0254 | 8 |
| 300 | 0.0003 | 0.0076 | 0.0006 | 0.0152 | ±0.0015 | ±0.0381 | n/a |
| 500 | 0.0005 | 0.0127 | 0.001 | 0.025 | ±0.002 | ±0.050 | n/a |
| 1000 | 0.0010 | 0.0254 | 0.002 | 0.051 | ±0.005 | ±0.127 | n/a |
| 2000 | n/a | n/a | 0.005 | 0.127 | ±0.005 | ±0.127 | n/a |
Table 1
5.2. ISO 3290
| Grade | Sphericity | Variation of ball diameter | Variation of ball lot diameter | Surface finish |
|---|---|---|---|---|
| G | μ metres | μ metres | μ metres | Ra Max. µ metres |
| 3 | 0.08 | 0.08 | 0.13 | .010 |
| 5 | 0.13 | 0.13 | 0.25 | .014 |
| 10 | 0.25 | 0.25 | 0.5 | .020 |
| 16 | 0.4 | 0.4 | 0.8 | .025 |
| 20 | 0.5 | 0.5 | 1 | .032 |
| 24 | 0.6 | 0.6 | 1.2 | .040 |
| 28 | 0.7 | 0.7 | 1.4 | .050 |
| 40 | 1 | 1 | 2 | .060 |
| 60 | 1.5 | 1.5 | 3 | .080 |
| 100 | 2.5 | 2.5 | 5 | .100 |
| 200 | 5 | 5 | 10 | .150 |
Table 2
5.3. DIN 5401
| Grade | Sphericity | Variation of ball diameter | Variation of ball lot diameter | Surface roughness |
|---|---|---|---|---|
| G | μ metres | μ metres | μ metres | Ra Max. µ metres |
| 3 | 0.08 | 0.08 | 0.13 | .010 |
| 5 | 0.13 | 0.13 | 0.25 | .014 |
| 10 | 0.25 | 0.25 | 0.5 | .020 |
| 16 | 0.4 | 0.4 | 0.8 | .025 |
| 20 | 0.5 | 0.5 | 1 | .032 |
| 28 | 0.7 | 0.7 | 1.4 | .040 |
| 40 | 1 | 1 | 2 | .050 |
| 100 | 2.5 | 2.5 | 5 | .060 |
| 200 | 5 | 5 | 10 | .080 |
| 500 | 25 | 25 | n/a | n/a |
Table 3
5.4. Ball size, grade and material table
| Material | Smallest standard size | Largest standard size | Best standard grade | Lowest standard grade | ||
|---|---|---|---|---|---|---|
| Stainless Steels | mm | inch | mm | inch | G | G |
| 302/304 | 1.5 | 1/16 | 50.8 | 2 | 50 | 1000 |
| 316 | 1.5 | 1/16 | 50.8 | 2 | 50 | 1000 |
| 420 | 1.5 | 1/16 | 50.8 | 2 | 25 | 1000 |
| 430 | 1.5 | 1/16 | 50.8 | 2 | 100 | 1000 |
| 440c | 1 | 1/32 | 50.8 | 2 | 10 | 1000 |
| CorrTech Steel | 1 | 1/32 | 50.8 | 2 | 10 | 1000 |
| Other materials | mm | inch | mm | inch | G | G |
| Chrome | 1 | 1/32 | 50.8 | 2 | 10 | 1000 |
| Low Carbon | 1 | 1/32 | 50.8 | 2 | 100 | 1000 |
| Brass | 1.5 | 1/16 | 25.4 | 1 | 200 | 1000 |
| Tungsten Carbide | 1.5 | 1/16 | 50.8 | 2 | 10 | 100 |
| Borosilicate Glass | 2 | 3/32 | 31.75 | 1.25 | 100 | 1000 |
| Soda Lime Glass | 2 | 3/32 | 31.75 | 1.25 | 100 | 1000 |
| Plastic | mm | inch | mm | inch | G | G |
| Acetal (Delrin and Celcon) | 1.5 | 1/16 | 25.4 | 1 | G0 | G3 |
| Nylon | 1.5 | 1/16 | 25.4 | 1 | G0 | G3 |
| Polyethylene | 1.5 | 1/16 | 25.4 | 1 | G0 | G3 |
| Polypropylene | 1.5 | 1/16 | 25.4 | 1 | G0 | G3 |
| PTFE (Teflon) | 1.5 | 1/16 | 25.4 | 1 | G0 | G3 |
Table 4
If you have any questions (or additions and corrections), please use the BikeGremlin forum:
www.bikegremlin.net
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.
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.
Hi Tommy… It is Andy your old Fraternity Brother…
Where to find size of bearings for Fuji S10S 1974 used for the bottom bracket (crank bearings)?
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.
surface roughness
wawiness
egg shapeness
thought I learned a new word today…wawiness
Thank you for the heads up.
Please let me know if you find any other “bid spilling”. 🙂
This is very helpful and you have solved my problem of what size my bearings are so I can buy replacements.
Thank you. Stuart
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…
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).
Thanks for summing all this up nice and succinctly. Most useful article I’ve read all day!
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.
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.
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?
Thanks.
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.
Relja
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
Thanks Mike and Relja for your replies, I shall discard the cages.
Good article, but 3/32 is 2.381, not 2.281mm 😉
greetings from Poland
Hi Lukasz,
Thank you very much for the correction. If you notice any other mistakes, don’t hesitate to not them. It’s appreciated. 🙂
Relja
What about stainless steel bearings for hubs? Are they better then regular?
Hi Mantas,
I don’t think they are better for bicycle bearing use.
Grade 25, 52100 chrome steel (not chrome-plated) is as good as it gets (and makes sense in terms of cost-benefits) for the application.
i use enduro grade 25 bearings they are the best ones,any harder bearings will only wear down the cones,the grade 25 ones last a very long time
Wow, great website. I knew nothing about bearings but wanting to start servicing my old GT bike myself, this website has helped me understand everything I need to know. Thanks!
Thanks Relja for taking the time for putting this information together! Very detailed and helpful!
Hi Relja
Would the following be ok for a Shimano rear hub?
https://simplybearings.co.uk/shop/Cycling-/-Biking-Bike-Bearings-Loose-Balls/c5409_5413_5415/index.html?selection=AISI+52100+Chrome+Steel+Loose+Ball+Bearings
James
Hi James,
Those are G100 bearings. I would prefer G25 quality.
However, it is fair to say that I have used G100 without any problems (except that the bearings don’t feel as smooth, though that is very subjective).
Hi Relja
I will go for the G10 option then, they don’t seem to have a G25 option.
Thanks
James
Hi James,
I think that’s a good idea – and that is exactly what I’ve been using over the past… almost 10 years.
The only decent-quality bearing balls I can source locally are G10. They are still a lot cheaper than “Shimano original” bearing balls and work without any problems (both on my bikes and on the… not sure how many bikes I’ve serviced over the years). 🙂
Thank you for your great article.
I intend to buy G20, 52100 chrome steel bearing balls. According to the online store, their hardness is between 60 and 66 HRC. Are they too hard?