[Closed] Shimano XT crank keeps falling off. - is the little plastic bolt necessary?

I've found it almost impossible to get the crank on tight enough without using the pre-load thing - every time I've tried, well... the arm has fallen off.

Sadly, I've found Shimano crank are never the same after they've come loose in use and will come loose from time to time afterwards.

That plug is just for preload. I'd say if the cranks are not staying on with the bolts tightened then worn splines? Time for new cranks. Have you been over tightening? Using a torque wrench?

The plastic preload "thingy" is to preload the bottom bracket bearings (so they spin, rather than scrape around!) NOT to preload the crank arm to the crank axle!

If the crank arm is coming loose from the splines you have a significant issue somewhere!

Similar experience to P-Jay here, cranks came loose and I lost the preload bolt. Bought another bolt, but the cranks came loose again soon after and I lost the new preload bolt. I just replaced the cranks after that.

Surely the little plastic bolt is just there to preload and position the crank arm in the correct place before tightening the bolts, you would have thought if it served a higher purpose it wouldn't be made out of plastic?
You would think that the crank arm should stay in place without it

You're right. There's something else wrong with your cranks. The alloy version may help but it might also just mask the issue until a point where the crank falls off in the middle of nowhere...

you aren't getting the arms all the way on. I have an old set that started to work loose regularly. Cleaned up the splines and the mating interface and it seems much better still have to use the lockring to make sure its properly seated.

In theory you could take it off once they are seated.

Alright pay close attention. I've had exactly the same problem as you before and it was driving me nuts.

You're right about the preload bolt. It's only purpose is to preload the crank so it can be tight enough before you do the main arm bolts. It does NOT hold the crank on by any means.

The fact that your crank arm gets undone means that it does not have enough space to properly hold itself on the shaft.

Let me take a wild guess here and say you probably have one extra spacer on there. If you have a 73mm bb you only need ONE spacer on the drive side. If you have a 68mm (or whatever the smaller one is) BB then you need TWO spacers. One on each side.

In my case, the dude that put the bike together had made a mistake and used TWO spacers on a 73mm BB. That resulted in the crank arm being pushed ever so slightly outwards, enough for it to not maintain grip properly.

Check the length of your BB and how many spacers you have on. If you see a mistake, fix it and then do everything up again using the new preload bolt that you ordered. The crank should not fall out any more.

Hopefully there is no damage done to the grooves of the arm and you can just do everything up normally and ride on!

sounds like the spline on the crank arm might be damaged, if you need a new crank arm its quite expensive for xt (£60 a few years ago).

You got the little pin that goes between slot mine kept coming off as the little metal pin had snapped

In theory you could take it off once they are seated.

Don't try this, once the crank bolts are tightened it's not coming off!

But yes, it's not for keeping the cranks together.

Alright pay close attention. I've had exactly the same problem as you before and it was driving me nuts

You got the little pin that goes between slot mine kept coming off as the little metal pin had snapped

That's true. It snaps off if the crank comes off. Mine did as well but after putting everything back together properly, it did not matter at all that I didn't have the pin any more.

Me too now I use sram or race Face e cranks and all that stupid shimano stuff doesn't bother me. Take a look at the alternatives.

This was due to user error and it has nothing to do with Shimano cranks. In fact Shimano make some of the best cranks and really value for money. I'd hardly ever look elsewhere to be honest.

B.A.Nana - Member

sounds like the spline on the crank arm is probably damaged, you need a new crank arm. quite expensive for xt (£60 a few years ago).

I've seen SLX arms for sale on CRC, but they're the older ones and oddly they don't (or didn't) list the arm length - anyway as you said about the XT one - they're £50...

Alternatively to paying £50 for a crank arm that might fix the problem, you could buy a completely new SLX crank, which comes with rings, bolts, a shiny new BB for the princely sum of £60...

It's a cotic soul that I built myself and I masured the BB shell (thrice) and followed the instructions with the BB, one spacer iirc.
I haven't got a torque wrench so my guess is that yes I have been overtightening.
Any good deals on Shimano cranks about? lol.

edit: thanks pjay.

In fact Shimano make some of the best cranks and really value for money. I'd hardly ever look elsewhere to be honest.

Me too now I use sram or race Face e cranks and all that stupid shimano stuff doesn't bother me. Take a look at the alternatives.

[b]This^^[/b]

Alternatively to paying £50 for a crank arm that might fix the problem, you could buy a completely new SLX crank, which comes with rings, bolts, a shiny new BB for the princely sum of £60...

Another thing to note: After all this crank falling off stuff because of the extra spacer, my crankarm was not damaged and I did not have to replace it. It has never fallen off again in 2 years.

I checked the Cotic site and yes your frame has a 73mm BB so you should only have one spacer on the drive side and NONE on the left side. Check again just to make sure.

And to add needing to search for the little pre load tool, guess the pre load, nip up bolts check pre load etc. Bin when the splines are worn... I've used loads of htII cranks and in the end I'm converted to the rest. I know not liking shimano in here is frowned upon but there are merits to the other designs.

There's no preload tool needed for the original plastic one. A wide knife can do the job. You dont need to do the bolt tight anyway. Just wind it in until the arm touches the end.

As for the alloy replacement ones, they're usually HEX so it's even easier. Just be careful not to do the bolt super tight.

It's literally super easy to install properly when you know what to do.

Fair enough try fitting a sram one. Then dropping the spider and direct mounting a single chain ring.

[i]Let me take a wild guess here and say you probably have one extra spacer on there. If you have a 73mm bb you only need ONE spacer on the drive side. If you have a 68mm (or whatever the smaller one is) BB then you need TWO spacers. One on each side.[/i]

Both on the drive-side.

And it doesn't matter where your spacers are if you have the right number.

Chain line.

Well it depends on what kind of chainline you want and how many rings you run.

But in general you're right. You're free to move the spacers around as you like.

Let me take a wild guess here and say you probably have one extra spacer on there. If you have a 73mm bb you only need ONE spacer on the drive side. If you have a 68mm (or whatever the smaller one is) BB then you need TWO spacers. One on each side.

It's 3 spacers on a 68mm - normally 2 drive side, 1 none drive. Perhaps you're thinking of Sram GXP where its a spacer each side for 68, and none for 73.

I would have agreed with everybody else that the preload bolt only preloaded the bearings until this happened to me. So how the crank arm fell off and this damaged the preload bolt, the crank arm kept falling off after this so I took the preload bolt off my other bike and bike 1 was now fine. I went out a couple of days later on bike 2 and the crank arm fell off. God knows why as I hadn't undone the arm bolts to remove the preload bolt. A new preload bolt fixed the issue. Both cranksets were fine, one is still in use by me on my BFe and the other has been given to a mate. I had no further issues and the correct amount of spacers were used with both BBs.

I can't really explain why this happened all I know is that it did.

You're absolutely right. I got that one wrong. It is three for 68mm.

Mine was 73mm so I didn't remember the other one correctly.

Too bad I can't edit that post to correct the mistake.

Thank you for pointing it out!

Im surprised you COULD take the preload bolt out with the crank firmly bolted on!

I've just mangled and chucked my XT drive side spider/crank due to under-tightened chainring bolts.

As a consequence have a spare non drive side crank with preload screw but the little locking lever is broken, about 4 years old, you're welcome to it for postage if you wish. I can't throw away stuff that's not broken! though sounds like a new preload screw will fix it.

Me too now I use sram or race Face e cranks and all that stupid shimano stuff doesn't bother me.

If you're having trouble with slippage do em up a bit tighter, tightening properly (both at once or each a bit at a time) and getting the two pinch bolts to equal torque/tension. If you're till having trouble then I'd probably try cranking em up to silly levels as the NDS crank is probably shagged anyway.

bforbertie if OP doesn't want it I'd be interested (mail in profile) got shimano cranks on a few bikes, I wouldn't mind a spare and I'm not precious about having odd cranks 🙂

The crank arm WILL NOT stay on without the preload cap no matter what. And once you have tightened it up without the preload cap it'll never be the same as mentioned above. weird, but this really is true.

just get a new bike

I always noticed that, with the preload tightened up correctly, the crank bolts would tighten onto this, instead of purely onto the axle. Whether this creates a tiny bit of misalignment which could give the cranks a chance to work loose and knacker the splines over time, I don't know.

Now I tighten the preload, tighten the crank bolts enough to stop it moving, then back off the preload, properly tighten the crank bolts, then adjust the preload back in again.

I've not had a repeat of the wrecked splines and random crank bye-bye I had with my first HTII crankset!

Either way, OP's crankset is probably toast.

Ironically, my XT cranks which have been fine for six months, fell off this evening.

Anyone spotted any good deals on cranks?

I had same problem and spent time and money, I ended up buying the cap but in metal, and it's never fell off since