42mm ISIS bearings- Milling them down

'Sup Hooommmieezsss

Just a question for the techies and machinists/engineers out there:

I’m wanting to reduce a 42mm ISIS bearing to an OD of 40mm. Essentially shaving/milling the outer diameter ring down by 1mm across the whole diameter. Is this doable and still function fine enough? They’re not going to be under excessive clamping forces or anything, they’re literally just going to pop into a lollipop bearing holder and stay in there.

If I want to go about doing this, is it a simple enough job for a machine shop/engineers’ to do it to just 2 bearings?

Cheers! :slight_smile:


You might be better off milling the bearing holders out to 42mm. 1mm seems like a lot of metal to take off a bearing.

Thought about that, but definitely not doing that. I’d have to mill down lollipop-inseration tabs on my frame for it to line up with the bolts properly. In future I may even switch back to Profile so I’ve want to stick with 40mm all the day.

Still, would 1mm shaved off the outer diameter of the bearing be alright?

Seconded, I would have thought the bearing would collapse easily with that much material removed.

Aren’t there any bearings on the market (not just the unicycle shops) that fit the ISIS axle and have an outer diameter of 40 mm?

I haven’t seen any production bearings with those dimensions. The best bet would be the KH custom bearings, which I don’t like very much.

I may be mistaken, but I believe the races in cartridge bearings are hardened. If that’s so, it may not be possible to machine them without cracking them. I would talk to a machinist and find out if they can be turned down at all.

I know that the outside thickness appears to be thick enough to be trimmed down, but the inside will certainly be thinner in the middle of the race. I would take one apart and measure the thickness, or better yet, find an exploded dimensional drawing of the bearing in question and see what the spec’d thickness is in the center of the race.

You know, you would think so

Having worked on many brands of motorcycles for many decades, I thought I knew something about bearings. I didn’t go to the Honda or BMW dealer etc,to buy their bearings with the correct brand number. Napa, or any bearing supply shop had the right part. They were not brand specific. Millions of vehicles were made by multitude of brands world wide, without the need to make a unique bearing size. Why would uni’s be different?

They are. Older Torker Dx’s had a unique bearing. My '05 KH has a 20x42 mm. One of the most common bearing sizes on earth. My '07 has a 22x42 , as do all ISIS uni hubs.

I guess that when they (all uni makers) went to the ISIS standard, they decided to invent a screwy new bearing , rather than retool all their frames to accept a standard 22x44 mm size. And even 22x44mm is a screwy rare size, just not rare and screwy enough to make unicycles.

Here is a link to a good bearing supplier. Don’t bother trying to find a 22x42 ISIS bearing. UDC is the place to look for ISIS bearings.

Sorry to go on such a long rant…, I just like bearings, except I hate ours.
To the OP, no, I don’t think that will work. If you chuck up the bearing in a lathe and spin it , the tool will not cut the outer race because, well, it’s a bearing. Should you surmount that fact, I still vote it’s a bad idea. With a diamond you could cut the bearing shell ( hard steel) , but that would weaken the part to much.

I think that the machine shop won’t be able to do it; they are hardened, and it will be a major PIA. Let’s say it takes a machine shop 2 hours to do it; that could easily be $100 or $200. It might be easier/cheaper to buy another frame.

Instead, you should turn the inside diameter of your bearing holders; that is more doable by a machine shop. Still, it would be expensive.


Bearings are case hardened, in that only a thin layer of the outside is hard. getting through this is easy as most well equipped tool shops will have ceramic cutting tips to do this.

Holding the bearing in a lathe however, is a different, much harder problem.

Once you remove the case hardening, you will weaken the bearing signifigantly, and it will fail in short time.

Last problem is there is less than 1mm of material around the thinnest part of the shell (the grove where the berings sit) so it would split in half anyway, if you try to take that much off. 1mm is a lot in the machining world.

I assume you are wanting to put these in your Ti frame, so getting that machined will be difficult anyway as most tool shops will struggle with titanium.

I would go for the KH bearings, even though I dont really like them either. They seem like the best of a bad bunch though in this case.


Hello All,
Well Bearings that are of Quality Have Heat treated races that are hard all the way through-Period. and to reduce the outer shell by .03937" (1mm), You would have to grind the outer race about .019" (.5mm) about as thick as four sheets of notebook paper. It can be done, I am not proud of it but I have done this horrible thing with no consiquences. But no one has stated the width? On my odd but special Muni’s I run more than one bearing on a side! So industy standard bearing number 6904 is I.D. of 20mm and an O.D. of only 37mm but the width is only 9mm. The #6804 is 20mm I.D.-32mm O.D. x 7mm wide. So with that you could run two or three side by side in a split piece of tube the entire width, with a wall thickness of say 1.5mm for the 6904 or 4mm for the 6804 so thats close to 1/16" and 1/8" respectively so what are your thoughts on this method?

   Cost out the door from McMaster Carr $8.85 each-both numbers

Simple answer: No.

The more detailed one: The structure of a bearing is calculated on a set of formula which is created by a series of tables based on forces that correspond to a thickness of bearing. This formula not only gives the thickness of the outer and inner bearing cases but also the size of the bearings. The bearings are then considered to be standard and you can go to any manufacturer and have them made then cross reference it against a table that tells you the force and fit that should be used when applying them to the shaft and what the maximum weights it will take.
Now… our unicycle ISIS bearing. If it was constructed to the standard table it would have an outside diameter of 47mm! If we did this we would have massivly heavy clumsy looking bearing houseings. It has been made by reducing the thickness of the inner and out bearing casing. This was done knowing that they will not be used at high speed and that they will need to have a reduced slip fit tolerance range. You may not realise but the bearings have developed over the 6 years that they have been in existence. I am sure you all know of the problems that are seen with Koxx bearings breaking on the outer casing where it has been reduced to thin. The latest batch bearings have a tighter tolerance on the inner casings, new seals and bigger bearings fitted in a deeper groove… this is the continual development that goes on in our sport in the back ground.
Now back to the problem. Removing 1mm from the outer shell. Yes it is physically possible to do but I will be surprised if it lasts more than a few days.
We do actually have a bearing that can do this job. The KH 40mm ISIS bearing.

This bearing uses a twin set of bearings. This is so that the height of the bearing is reduced by using a smaller bearing and bearing case. The twist and weight loading of the bearing is helped by having a twin set of bearings but there are disadvantages… it does not spin as freely, it has even tighter tolerance on application – basically it is intolerant to misuse on fitting. (I know from experience that it is hard to fit correctly, once right it is good, but it soooo easy to damage).
Sorry about the rambling, I hope this helps explains.