Crank arm issue.

[Tabs2000]

The crank arms on my Decoy base model were creaking so I called YT up got the torque specs and tightened them up accordingly.

I had just checked them yesterday and today while on a ride the right side crank arm fell off.

The splines seemed ok so I did a quick trail side repair and road the bike home. The center lock ring is broke at the flange and I’m thinking that let the crank arm slide off. I have been searching the internet for this part but cannot find it? Does anyone have a clue as to where I can buy this part.

I sent a request to YT with photos but it’s driving me nuts.
here is a photo of the part.

 

[Gary]

The damaged part is not a lock nut, it's simply a pre-load cap.
it's not torx either. it requires a shimano hollowtch tool to adjust pre-load tension when fitting the arms.
You can buy a replacement pre-load cap from any bike shop with a shimano account (pretty much all shops). But do yourself a favour and replace both driveside and non driveside pre-load caps with aluminium ones. Either fit Shimano Saint aluminium caps or cheap generic chinese allen bolt versions.

What has happened is as your cranks flex the arm has started "walking" off the splined axle, the plastic pre-load cap threads and the shimano safety tab between the pinchbolts are not strong enough to hold the crank on once loose. Fitting aluminium caps as I described will help hold the crank in place and reduce the flex and chance of the arm "walking". but even with Alu caps fitted, periodically check the gap between the crank and motor has not increased and check pinch bolt tightness. A little (blue) threadlock on both won't hurt either.

 

[KenX]

Great service from this guy:
Crank Arm Fixing Bolt For Shimano HollowTech II HTII Crank Chainset CNC M20 | eBay

 

[KenX]

Don't forget to order 2!!!

 

[Tabs2000]

The damaged part is not a lock nut, it's simply a pre-load cap.
it's not torx either. it requires a shimano hollowtch tool to adjust pre-load tension when fitting the arms.
You can buy a replacement pre-load cap from any bike shop with a shimano account (pretty much all shops). But do yourself a favour and replace both driveside and non driveside pre-load caps with aluminium ones. Either fit Shimano Saint aluminium caps or cheap generic chinese allen bolt versions.

What has happened is as your cranks flex the arm has started "walking" off the splined axle, the plastic pre-load cap threads and the shimano safety tab between the pinchbolts are not strong enough to hold the crank on once loose. Fitting aluminium caps as I described will help hold the crank in place and reduce the flex and chance of the arm "walking". but even with Alu caps fitted, periodically check the gap between the crank and motor has not increased and check pinch bolt tightness. A little (blue) threadlock on both won't hurt either.

Thank you so much. I’m scheduled for shoulder surgery may 6th and I thought the bike was going to be out of commission until after I recovered. I will head out and grab some today, I think there are a few bike shops open around me. Hopefully they will have the tool also.
thanks so much cheers,
tim

Edit. I found a couple saint pre load caps at my local bike shop. I ordered and paid online and they open at 10 am. It looks like I will be riding today! Thank you so much for the info.