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Stripped out crank arm
#1
I got a bike about a week ago for $15. I was going to use it for parts (for another bike I have), but after examination (and because the rims were the wrong size), I've decided to repair it instead.

Apparently, this specific model crank set has the crank arms separate from the gear assembly. And the left crank arm had stripped out the thread and square peg which it had been fastened to. The entire rest of the bike is in excellent condition, though I'm planning to replace the brake pads.

[attachment=4577]

My idea is to drill a hole through the peg straight into the crank arm, liberally apply J.B. Weld over the whole area, and force a nail into the hole I drilled. If it goes like I plan, it should create a very strong joint.

Has anyone ever encountered a problem like this before?
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#2
This is a common problem. I would recommend just buying a new "bottom bracket" (bearing assembly) and a new crank set or at least a new left crank arm. Or find another parts bike to strip.

Your concept may work for a while, but I'm guessing not that long. This is a very high stress area and you typically have a steel axle with aluminum crank arm. Drilling holes in it will make the whole thing prone to snapping off and possibly injuring you. JB Weld is also basically epoxy. Despite the name, it has nothing to do with "welding" and none of the strength implied by the name. I've seen crazier schemes work, but I wouldn't ride it...
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#3
(10-21-2013, 05:07 PM)DaveM Wrote:  This is a common problem. I would recommend just buying a new "bottom bracket" (bearing assembly) and a new crank set or at least a new left crank arm. Or find another parts bike to strip.

Your concept may work for a while, but I'm guessing not that long. This is a very high stress area and you typically have a steel axle with aluminum crank arm. Drilling holes in it will make the whole thing prone to snapping off and possibly injuring you. JB Weld is also basically epoxy. Despite the name, it has nothing to do with "welding" and none of the strength implied by the name. I've seen crazier schemes work, but I wouldn't ride it...

When I test-fitted the crank arm, it fit very snugly (no looseness at all). I'm not sure why the previous owner thought it was broken.
So, instead of drilling the hole, I simply smeared the entire area with J.B Weld to help prevent any minor slippage that may occur, and then bolted the crank arm down snug. After letting it set for about 30 minutes, I took it out for a test-ride. The whole bike works perfectly. Checking it after the ride, there was no looseness or any signs of stress.
I will check it from time to time to make sure it's holding, but as of now, I'm pretty confident it's a permanent fix.
  Reply
#4
it is your bike......

just don't sell it or loan it to anyone; Liability Lawyer would love the $$$$$$.
Nigel
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