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I recently had to dash home for a minor emergency. I noticed that one pedal was a bit wobbly but I had no choice but to push on. Once I got home and the emergency had been dealt with I noticed that my pedal no longer screws into the drive side crankarm as the rotating of the pedal when it was wobbly has worn away the grooves within the crankarm.

What should my course of action be? Do I need to replace the crankarm or as its the drive side do I need to replace the whole chain set?

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Yes, you need to replace the crank arm, if tightening the bolt does not make it 100% solid again. No doubt the crank arm had been loose for days, and had you tightened it earlier you might have "saved" it.

And there's some danger that you have damaged the crank axle as well, meaning the bottom bracket cartridge will need changing out as well.

I have speculated that a slightly misshapen crank could be saved by fashioning a shim of aluminum sheet metal (ie, beer/pop can), but I've never had need to try that, and it sounds like yours is in too bad of shape for that to have a chance to work.

(I'll add that replacing just the arm vs a larger assembly is a matter of feasibility and economics. If you can't find a compatible replacement arm you may need to just install a new crankset, or you may discover that the entire crankset is cheaper than the replacement arm.)

Added

As others have pointed out below, the OP's description is more consistent with a failure at the joint between the crank arm and the pedal than between the crank arm and the crank shaft. If this is the case there is some chance that a "Helicoil" can be installed to fix it. This can be done by a skilled bike mechanic or a machine shop.

(In this case the pedal is likely toast, however.)

  • You're totally correct about being able to save it had I done something earlier, it's incredibly frustrating! Ah well, I can't cry over spilled milk. There's no chance of restoration I'm afraid, it's turned from a rifle barrel to a smooth bore musket. – MyFamily Feb 3 '15 at 12:43
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    It should be noted for completeness that you can sometimes do a repair (parktool.com/blog/repair-help/pedal-bushing-kit), but likely you'll have to replace the whole crankset because you likely wont find just the arm you need. – Batman Feb 3 '15 at 14:01
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    It sounds from the question that the problem is at the pedal/crank interface not the crank/BB interface ("my pedal no longer screws into the drive side crankarm") in which case the BB/crank axle is unlikely to have been affected. You'll probably have to get new pedals and/or cranks (for higher level ones you may be able to just replace the damaged one). – Holloway Feb 3 '15 at 16:07
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    @Trengot - I missed that. If it is indeed just the pedal threads that are damaged (not the junction with the crank shaft) there is some chance that having a "Helicoil" installed can fix it. – Daniel R Hicks Feb 3 '15 at 16:33
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    @MyFamily - Very likely the threads on the pedal would be fouled (or, even, it's a left-hand pedal forced into the right side), and attempting to reuse it would muck up the Helicoil. You could have the mechanic carefully inspect the pedal then try to thread it in, but care would be needed, and he should stop the attempt as soon as there is a hint of resistance. – Daniel R Hicks Feb 4 '15 at 12:44
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Are you pretty good mechanically? You could try this: Repairing stripped pedal thread

  • Excellent answer and a very interesting article. Unfortunately I'm not good enough to attempt that myself, nor do I have the appropriate apparatus. – MyFamily Feb 4 '15 at 17:41

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