Square axle bottom-bracket bolt torque?



C7ZZGT

New Member
Sep 10, 2006
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My and I'm already looking for help. Go figure.:confused:
I have a square-type Shimano bottom-bracket axle and cranks. I have removed the cranks, greased the axle, (tried the cranks un-greased too) and then put them back on only to have a clunk type sound emanate from the BB at the same place while cranking. When I return home I find the bolt holding the cranks on have either loosened or the cranks have moved further in on the axle. I have used a torque wrench set to 28 lbs-ft each time but after a 50 minute ride I find I have to re-tighten the bolts.
Question: What should the torque be for these bolts and am I missing something obvious to you'all?
Thanks!
 

Retro Grouch

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Dec 29, 2005
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You're about right with the amount of torque but I fear you're a little too late.

Once those square taper cranks loosen up, continuing to ride on it it tends to wallow out the square hole in the relatively soft aluminum crankarm and they're ruined forever.
 

C7ZZGT

New Member
Sep 10, 2006
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Thanks. I'll try to torque them one more time - with some blue loctite on the bolts - at the high end of the torque range: 36 lbs-ft.
If that doesn't work, I guess I need some new crank arms...?
Again, thanks!
 

ScienceIsCool

New Member
Jun 25, 2006
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C7ZZGT said:
Thanks. I'll try to torque them one more time - with some blue loctite on the bolts - at the high end of the torque range: 36 lbs-ft.
If that doesn't work, I guess I need some new crank arms...?
Again, thanks!
You might need new crank arms... I agree you should try re-tightening first. Why not? You've got nothing to lose.

The square, tapered bottom bracket shaft/crank arm is designed to be an interference fit. That is, once tightened to the proper torque spec, the aluminum crank arm should deform slightly and make a snug, full contact fit with the spindle. It should even be possible to take out the crank arm bolt at that point - though you should leave it in, in case the crank arm comes loose. The bolt will keep it from falling off!

It's recommended that you don't grease the axle or crank arm prior to installation since this will mess with the interference fit. Also, once the crank arm has come loose and been ridden this way, the soft crank arm will probably deform a bit. This will also wreck any future attempts at re-tightening and making a good interference fit. I think this is the situation you're in right now.

Try re-tightening. Just don't tighten past the recommended specs. It's possible to over-stress the crank arm and have it crack/fail.

John Swanson
www.bikephysics.com
 

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