Busted Crankarm
 

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So I took my shiny new Shimano Tiagra Compact crank+performance platform pedals for a spin and then after a few km of riding after a trackstand on a red light I noticed that my right pedal(ugh! why does it have to be the expensive side!) is loose. I figured it might've magically unscrewed itself, but it was loose and at an angle. So I wiggled it out and along came the threads :(

Where did I go wrong here? I've replaced pedals many times before on my other cranks, but i've never used this crank or pedals before. Now that I think about it, the pedal did start going in too tight to spin in by hand about halfway through(ie first half went in smoothly), but I figured I can apply a little bit more force with a pedal wrench.

Also I just noticed that the threaded part of the left pedal does not go all the way through. There is a fair bit of room remaining. Are the pedals to blame?
Is there a way to get the right crank arm cheaply? I paid $50 for these cranks, but i'd rather not throw out a mostly brand new crank :(

is it possible I put too much grease in? Any other theories?

Looks like a mess to me. I do not think that you can get away cheaply and will probably have to replace the right hand crank arm with the same model, or better one. It is always best to put a light amount of grease on the pedals when you screw them in to the crankarms. Althernatively, a bit of light oil. I have never heard of putting too much grease on the pedals, and even if you did, this would not cause the pedal to back out. If you have a good independent bike shop around, you could attempt, and I mean attempt,to rethread , or tap , the crank arm and see if the pedal would thread back in. Generally, the pedal shaft is carbon steel and the crank arm is alloy and weaker than the pedal. You have to ensure that you thread the pedals into the crank arm straight also. Good luck.

I'm not saying that it would be OK for your application, but I've used Helicoil threaded inserts in the past for other jobs and had good luck with them.

when i worked at a shop we had a helicoil kit for crankarms, and another mechanic used it a few times with good results. i also saved the oil pan on my BMW with a helicoil. but if the orignal poster has to buy an entire helicoil kit it is going to cost about $100...thats what the one for my BMW's oil drain plug cost....

Thanks. I'll try a bikeshop. if I can get helicoiling done for under $40, it beats buying another new crank. Seems that buying the kit myself will cost more than a new crank(or similar).

the shop should not charge too much to do it, as long as they have the proper tools. all they have to do is thread it one size larger, insert the helicoil and install your pedal. some people like to use loc-tite on the thread insert when they do this.

Sounds to me like the cranks stripped when you installed them. Did you cross thread it?

Cheap pedals (platforms often are) have a different thread (1/2"-20) than better quality pedals (9/16"-20)

If you used these the fit would have been very loose so I don't know why you found dthem getting too tight to turn by hand unless they were cross threaded or installed at an angle.