I have a Nimbus 27.5 bought in 2021. I recently bought a V-frame Nimbus 36, age uncertain. I tried to swap the KH Moment cranks from the 36 onto my muni and the splines engaged 2/3 of the way and stopped. Then I tried the (KH spirit) crank from my muni on the 36 and it bottomed out with no tightening. Could there be that much of a discrepancy? I think I need more spacers if I want to try the Moments on my muni, but seems like a big difference in fit.
Sounds like wear on the Spirit cranks. Are the spacers on both unis the same and does the same thing happen when you try to seat the Spirits on the Muni?
Cranks are wear parts and, to prolong their life, you will occasionally need to move to smaller spacers until you have space between slid on crank and spacer again. This is usually needed when you remove / reinstall cranks but might also be because the cranks are making noises.
For the conspiracy side, I’ve pulled open my spares box and taken pictures of a brand new set of Spirits and a 1 install set of Moments. There are no spacers on the hub. While the intent was to measure using the ruler (which you can do if you choose), from eyeballing, the 2 sets slide on to a roughly similar point.
Almost every set of cranks I have sits slightly differently on my hubs, and did even when new. One set of cranks will also fit slightly differently on the range of hubs I have.
The ISIS interface needs to be made to very tight tolerances, and every manufacturer seems to do it very slightly differently. In theory I think there are ideal opposing ISIS interface gauges that you can use to check you’re making things correctly, but in reality I think they don’t exist/nobody has them.
The material cranks are made out of also changes how they deform when fitted, and once fitted once all cranks deform slightly and will be slightly looser the next time.
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davejh (we’re both Daves, apparently), thank you for going to all the trouble.
Yes, the spacers are the same. The same thing does not happen with the Spirits on the muni, which is why I’m reluctant to tighten the Moments down and possibly damage something. I took the other cranks off and tried the same thing on both sides; the moments slip on to the same point on both sides on the muni, and both Sprits slip all the way onto the 36 hub. Could it be something with the hubs? Or have the cranks on one of the unis been deformed from use? I don’t see anything obvious from a close visual inspection. Seems like if I wrench down and tighten the Moments onto the muni hub, when I go to return them to the 36 they’ll be looser. I’d rather not have to buy new cranks when I have cranks in hand 
OK, that sounds more like interface tolerance. I’ve attached the specifications for the interface below. According to the specifications a 0.175 (f) - 0.26mm (b) variation of the recessed part of the splines is acceptable. That recess is supposed to be a 1 degree slope so (assuming my maths is correct) a .175mm height difference is a 1cm seating difference!
Roger at UDC UK said that there is an agreement between unicycle manufacturers to work to a tighter specification (can’t remember if it’s hubs, cranks or both) but you could still easily have a different tolerance between the 2 with both being in spec.
Both cranks will be deformed from use with their previous hub, that happens as soon as you install cranks for the first time. Deformation gets worse with every install (hence needing smaller spacers) and ISIS cranks are only good for about 10 installs before the interface is worn out.
I suspect that if you install the Moments on the Muni, it would work fine but you would at minimum need to change spacers to return it to the Nightrider 36 if it worked at all. As both moments and Spirits are out of production (some Spirits still available new from UDC - UK/Canada definitely have) I’d consider replacing the Moments on the 36 with new cranks (multi hole Spirits or VCX+) if you decided to go that way.
Small mercies a previous owner of the Nightrider retrofitted an internal disc hub at some point so you’re not stuck needing disc capable Spirits for it!
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Thanks again for the response and helpful suggestions, you confirmed what I suspected and now I don’t feel crazy! The drawing is also very cool to see. It’s amazing how much engineering knowhow has to go into something as ‘simple’ as a unicycle. I will look into purchasing new cranks. I was not aware of the 10-install lifespan of a crank, that is also good to know. Cheers!
Some years ago I modelled the ISIS interface in CAD in order to make an ISIS to square taper adapter (no it was not a good idea, just a bit of fun) - it certainly took some head scratching to figure it out.
I used to work at a b!ke shop, and I pretty much never had anyone come in for new cranks except one guy … and, well … he was interesting. I never considered cranks a “wear item,” but I am not a year in riding my unicycle and I am on my second set. (It was because I’m dumb, not that I am athletic, hopped on it a while with loose cranks).
I learn so much about unicycle and bike hardware reading this forum!