M8 Crank Bolts for Unicycle.com 29"?

:grin: UPDATE: do you need is fixed, this issue is resolved! Thank you so much for those who helped me! I’m so grateful! :grin:

My new uni came! But it’s not all the way put together because there’s one hiccup:

I bought this Unicycle.com 29" road unicycle from someone (used, also slightly customised so it’s not entirely a Unicycle.com uni) and while I was putting it together, it seemed that the crank bolts would only screw in a half rotation and no more than that. I tried both M8x1mm and M8x1.25mm crank bolts. Does anyone have experience with crank bolts behaving like this?

Thank you so much! :grin:

Hi Cedar

The bolts on cotterless cranks should screw in freely quite a lot of turns. If the bolts you have are going in and turning 1/2 a turn check that they are the right thread (specifically that the bolt thread is not too coarse).

Do you have the same problem on both sides? Also can you stick something in the hole to see how deep it is? I would think it should be at least 20 - 25mm deep.

If you have the same problem on both sides it is probably the bolts that are wrong. If one side has a deeper hole perhaps there is part of a bolt broken off in it.

Don’t force these in, they should be free and go in a reasonable bit as it is the bolt that pulls the crank onto the taper. If you have any doubt you might be best take it along to a bike shop and let them have a look rather than damage anything.

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Looks like M8x1mm should be the correct size. With a good light and magnifier I’d very carefully inspect the threads for any problems.

Thank you so much for your response!

It is the same problem on each side, and neither has anything stuck inside them. We tried taking the bolts from another Unicycle.com uni and they wouldn’t work either. I think all the ones we tried are M8x1mm bolts, and also one M8x1.25mm too - could they be a different size?

My friend is a long-term bike mechanic, and he looked at it and did his best. Any advice would be greatly appreciated, thank you so much!

~ Cedar

I’m guessing you’ve got damaged threads. The first thing I’d want to do is get a set of thread pitch gauges. It’s extremely unlikely that the threads would be a different pitch, but you’ll find out for sure and you can rotate it around inside and feel if the threads are damaged. Hardware stores usually have them by the nuts and bolts and you can use them for free.

If the threads are damaged you might try taking it to a machine shop and see if they can clean them up with a tap or you could buy the tap and do it yourself.

Have you been testing with the crank on or off? If the crank doesn’t go on far enough or has some abnormality for some reason then could that prevent a bolt from going in? Check that the end of your spindle is not bent. I doubt that the portion of the spindle that fits in the crank could be bent (enough to prevent the bolt from threading in), but just a thought. If everything looks like it is in good condition and is the right part, then maybe make sure everything is clean and oiled/greased.
The threads in the hub don’t show any signs of being cross threaded or damaged in any other way?
Best of luck to you getting this figured out and fixed.

I would think these are all the same thread so the ones from the other unicycle should fit. I took the bolts out of a bike cotter less crank yesterday to have a look and they are quite fine threaded (which is why I mentioned checking to see it the bolts you had weren’t too coarse).

Given you’ve got another unicycle to try this on you can get an idea of what it should feel like on that. On your unicycle try to get the bolts to go in without the cranks on, you will see what is happening a lot better. Given you’ve got both M8 fine and coarse threaded bolts you will see there is a pretty noticeable difference between the threads, you should be able to see what the thread looks like in the hub to see which one looks correct – I would think that it is the fine one (M8x1)

I’m assuming you had to get this posted to you and the seller removed the cranks but didn’t send you the bolts? That seems a bit suspicious. You mentioned M8x1.25 bolts, are these just “standard” M8 bolts you got from shop – ie not specific cotterless crank bolts? “Standard” M8 bolts for just general bolting things together will probably not fit and if anyone has tried to force these in they may have damaged the end of the thread in the hole. In that case you can probably fix it by running a tap down it as mentioned above – take it to a bike shop to see if they can do that for you (I’m assuming these things aren’t heat treated/hardend and that they can be retapped easily). Even if the thread is damaged on the end at both sides the good news is that the damage should only be at the end and there should be enough good thread left for the bolts to easily take hold.

There is always the possibility that these aren’t metric threaded, but I doubt that. You could email or phone UDC in the UK and ask them, I would think these are all pretty standard though.

I have the same 29" UDC uni and I checked the bolts. They are M8x1 bolts, about 19mm long with a 19.5mm diameter flange.
Edit, it should be noted that M8x1 bolts are quite rare with most bolt suppliers not carrying that size. The M8x1.25 are very common but they will not fit.

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Hi Cedar,
If you come to the conclusion that the threads in your axle are damaged (perhaps the seller forced the wrong bolts in) and you can’t find anyone to tap this out for you, a couple of taps and a wrench are not all that expensive.

These were about the cheapest I saw on Amazon UK but should do the job (M8.x1.0). (£7.26 total for the taps and the wrench when I posted this). This has a starter and plug tap – you’d maybe get away with just using the plug given the hole is already tapped.

Search on YouTube for a video of how to use them if you or your friend are not familiar with these things – basically you screw it into the hole a little (say a 1/4 - 1/3 of a turn, then unscrew it a little, then screw in again, gradually working it back and forth a little at a time until you get it in as far as you want.

Tap wrench

M8x1.0 taps

Anyway, hopefully this helps.

Thank you so much for those who help me, this issue was resolved! I was able to ride it perfectly without any issues! Thank you so much for your help again!

What did it end up being?

We cleaned the crank bolts with another crank bolt that had slits in it. So when it was screwed in, it cleaned the threads. The first thread was destroyed but the rest were in tact and the crank bolts easily screwed in. The cranks are super secure now!


This thread also help me resolve my issue i am tired of this.

We cleaned the crank bolts with another crank bolt that had slits in it. So when it was screwed in, it cleaned the threads. The first thread was destroyed but the rest were in tact and the crank bolts easily screwed in. The cranks are super secure now!

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