ISIS crank stop sizing...

the crank stops on my nimbus unicycles seem to be consistent and roughly 8mm wide. Are there other sizes for different unicycles or cranks? Does the Kris Holm unis use the same size or different size crank stops? can you purchase different sized crank stops somewhere?

The size of the spacer you need depends on the type of crank you are fitting. For Moment cranks you need a 4mm spacer and ISIS cranks other than Moments you need 8mm spacers. sell the spacers

I build my own spacers with a 27.2mm KH seat tube and a saw.

It works fine and I can adjust exactly depending on KH, Tensile or QU-AX cranks …

That’s a nice idea, my spacers are pretty screwed up for my K1 light cranks…

Is a 27.2mm seatpost the right diameter to fit snuggly on the ISIS axle?

awesome! I was really looking for that and you pointed me in exactly the right direction! I hadn’t known about the right size spacers for Moments, and at one point, had Moments on all 3 unis with 8 mm spacers -oh no!

I wonder how this is determined, and if I need the 3 or 8mm crank “spacers” for the Sinz or AC cranks?

Nice! I may double check with vernier calipers… but great idea! 27.2 is a perfect fit?

I recently mounted 160mm Echo SL cranks on my Nimbus, which previously had Ventures and 8mm spacers. I had to shave a mm or two off the spacers to get the fit before screwing in the crank bolts to be similar to that with the Ventures.

I just distort a little bit the spacers made of 27.2mm seat post tube to make them very slightly oval in order not to loose the spacer when I remove the cranks

since the crank stops don’t appear to be available in the US, I think I’ll have to use your method.

Ok, aftermarket KH moment cranks don’t necessarily come with the smaller spacers, but I’m guessing that when they come stock on a KH Uni, that the 3mm spacers are stock. Anyone have a KH they can check?

I am wondering if spacers/crank bolts can be had from a LBS. Do crank bolts come in standard sizes?

bring all the pieces into the shop and you should get the right bolts. as far as Isis spacers, not likely.

Hmm. Does it mean I won’t be able to install my new shiny dual-hole KH cranks on a uni that previously had Ventures? Unfortunately there is very little information about spacer/crank compatibility on UDC and the US site does not even carry spacers… :frowning:

aren’t spacers just so your frame doesn’t flop around? (i know there’s a lot of talk about the cranks messing up if they go on too far, but are you really going to put 200 ft-lbs of torque on your cranks?)
if you just figure out where the crank stops when its snug and make the spacer ever so slightly smaller then that then everything should work out.

The spacers (crank stops) are there to stop the crank sliding too far onto the axle. On a unicycle hub they have the added advantage of holding the bearings still, but that’s not their main purpose as far as the ISIS interface goes.

ISIS axles are slightly tapered, but at a much shallower angle than a square-taper axle, so if you just do the crank bolts up really tight without a spacer/stop the crank will go much too far onto the axle and will always work loose easily (people have been having this problem with Schlumpf hubs, which have no crank stop and can’t use spacers because of the way they work). In theory you need a spacer just wide enough so the crank hits it when it is tight enough but not too tight, which can vary between different cranks. In practice (at least with the hubs and cranks I’ve got) it doesn’t seem to matter that much. Moments should use a smaller spacer than other cranks (4mm instead of 8mm I think), but I’ve got Moments on a Nimbus muni without changing the spacers, and Tensiles on a KH muni still using the narrow spacers. They all work fine, it’s not that critical.

Some people say you can run ISIS without spacers (just like a square-taper hub), but they seem to be the same people who say you need torque wrenches and check the bolts every ride, and still get cranks working loose. ISIS with a crank stop, as it should have, is just fit-and-forget. Do the bolt up until the crank hits the spacer, then just do it up quite tight by hand with and allen key - not as tight as square-taper needs to be.


I’ve never run spacers. I just tightened my tensiles up and left them. They seem fine, never worked loose on me. I didn’t realise you had to, just thought they were for super anal people, lol. I always chuck 'em in the bin :slight_smile:

What size spacers do we need for different ISIS unicycles? UDC is now selling replacement spacers. I have a Nimbus 24" with Qu-Ax cranks where the bearing have slipped on occasion. It appears this problem could be avoided with spacers. So, do I need 4 mm, 6 mm or 8 mm spacers?

Here is an example:

Is there a chart online that list appropriate spacer sizes?


I think the quick rule (on a UDC or KH hub) is 4mm for KH cranks, 6mm for others. But being really pedantic you’d have to take all sorts of things into account, including wear if the cranks have been fitted a few times. ISIS is a shallow taper and in my experience it doesn’t really matter that much. I’ve got some Moments fitted with 6mm spacers and some Tensiles fitted with 4mm spacers - both “wrong” but neither has given any trouble.

My 36er has a Nimbus hub with Qu-Ax cranks, and is currently running one 4mm and one 6mm spacer - the hub came with odd ones and I can’t be bothered to change it, works fine.


I posted this once, way back, about how I found that some cranks needed different spacers than others, depending on how they are machined. I think UDC uses 2-4mm (hub to bearing spacer) and 6-8mm spacers for cranks. I did find that the spacer provided with my KH and Nimbus hubs worked fine with the Moments, but when I got some K1 cranks I had to use a narrower spacer. I also had some fit problems with the Moments on the original KH Ti hub.

Having a variety of spacers on hand is a good for dong test fits. You don’t want it to loose of the bearings will shift out board and of the frame/bearing will flex too much. You don’t want it too tight or you’ll fry your bearing or causing excessive spline wear (wobble).

You’ll think twice about that after your bearing shifts on the axel and you have to walk home. Been there once, won’t do it again!