I bought this Onza trials second hand recently.
I noticed that on the one side there is a metal ring around the bolt attaching the crank to the hub, but on the other side this is missing.
Is this a problem? 
What is this part called?
and can it be bought so i can replace it?
Hopefully you can help! 
You’re missing a self-extracting ring.
It enables you to remover the cranks without a crank removing tool.
You can buy spares on UDC.
You can ride your uni perfectly well without it.
Thats ok then…no need to panic! 
Thanks
Not only can you ride it perfectly fine without it, but in many cases you can remove the cranks without it. The splines on the KH/Onza hub are straight without a taper, and so the cranks slide on and off. I might have an extra ring sitting around if you want it, but a set is pretty cheap at UDC.
Old KH & Onza Cranks
Both the Onza and KH 2005 cranks can be easily removed with just the correct size allen for crank rotations.
Unicorn
Andy,
Our friend “Into” is correct --that is a self extracting method to remove the cranks…and quite frankly is an ingenious method to do it. To remove the crank you back that piece and its washer (underneath) off from the hex bolt that holds on the crank arm, just a turn or so of slack. Then you back out the hex bolt that holds on the crank and the flange on that bolt pushes the crank arm off the axle without a crank extractor.
Both sides are identical (threaded the same way…unlike the pedals) and as a result, the one on one side of the unicycle (I forget which) is inclined to back itself out when you are riding and fall off. You really only need one because you can pop the crank off one side, then move the pieces to the other side and repeat the procedure to get the other crank arm off. I lost one and replaced it but when it came loose again, I decided to leave it in my tool box for a backup.
You really do not need either to ride the unicycle–they are only to remove the cranks. I have always been impressed with this ingenious method to eliminate the need for a crank puller.
Carey
Thanks for the information guys. Ill just leave it how it is for now, seeing as I have no wish to remove the cranks. And if I do in the future I can always get another set cheaply, or just use one as Carey pointed out!
@jtrops, thanks for the kind offer but like I say, I don’t think there is much point in replacing it right now.
If you do need to install or remove the rings, don’t bother forking out for an expensive tool.
Just use a pair of thin-nosed pliers as seen here.
Also, if you want to remove/install the rings remember that they are left threaded, and not normal right hand threads. In other words lefty tighty, righty loosey.