[Closed] Stripped XT non drive side crank arm

'kin hell - you must have legs like treetrunks.... or you didn't do the pinch bolts up 😉

When I've done this (not tightening the bolts up enough) the crank arm has broken the plastic pretension cap and slid off the spiles rather than rotating (que comedy double-take trying to work out why the crank is still attached to my shoe, but not making the bike move forward....). Are the pinch bolts actually still there/very, very loose?

Kept tight, I'd have thought they should last.

Pinch bolts were done up pretty tight, the pre-load plastic nut didn't come undone either, the cranks just rotated past where the should have been.

I undid everything at the side of the road and re-tightened it all but the splines on the crank arm looked completely shagged. It held together for 2 minutes but as soon as I put any pressure on the pedals it slipped again.

So train home tonight and I'll take the cranks off the bike when I get home and re-assemble them off the bike and see if they can be saved. But I'm pretty sure they are fubar.

Not the end of the world, sure I have a spare set somewhere!

If you're lucky, you can pick up a spare arm on Ebay for peanuts. Got a brand new Ultegra 6700 one for less than £4 the other day. Retail is a bit more than that.....

All sounds odd - I'd try and diagnose what happened so you don;t get a repeat any time soon (I accept this may be quite hard to do after the event just looking at the shagged bits and a pile of swarf....)

you can buy just a left arm as a replacement if the splines ont eh axle are ok.

This is actually quite common - the pinch bolts have quite a short amount of thread in the metal. You can usually 'fix' this by fitting longer bolts (replace both) as the thread goes much further than the standard bolt goes.

Clubber - I think the OP is saying that the splines to the axle are the problem....

though to the OP - might be worth checking the threads on the pinch bolts are OK - if the threads had partially stripped you may think you're doing the bolts up tight, but you may not have been clamping the crank onto the axle. Either way, sounds like a new crank arm.

Hmm yes, now I reread it, it does sound that way but I can't really think how they'd wear unless they were run very loose for a long time in which case surely you'd notice because the crank would be wobbly...

Pic?

if the threads had partially stripped you may think you're doing the bolts up tight, but you may not have been clamping the crank onto the axle

If they were stripped they'd keep turning!

Can't do pics until I get home. I'm sure its a combination of slightly loose old cranks and my massive leg power!

On a side note, I limped the last three miles to work not really able to pedal with any force, so I was going about half speed. Its amazing how poorly other traffic treats you when you are slow. No wonder a lot of people are put off cycling on the road!

I'm sure this happens because people don't tighten the bolts correctly. You have two bolts pulling on the same bit of metal so, as you tighten one bolt up, it loosens the other one. The tight bolt acts as a pivot for flex and the loose one lets the crank arm move a little, wearing the splines on every rotation of the crank.

You have to alternate between the two bolts, tightening them up a bit at a time until they're both fully torqued. You're meant to retorque them a couple of weeks after first fitting them, too.

If they were stripped they'd keep turning!

If you'd massively overtorqued it, yes. If you'd overtorqued it, but not stripped it, the threads will plastically deform in the crank - you can get the bolt out, but when it goes in next time the thread pitch is deformed from what it should be, so there is a lot more friction. It feels like you've done the bolt up (applied lots of torque), but most of that is overcoming friction against the damaged threads and not pinching the cranks.

I've done this myself.... 😳

EDIT: Shedfull - very good point.