Stripped threads on left crank on the first ride I just purchased a new road bike and had a friend assist me in the assembly. There must have been a problem because it fell off after only 2 miles of riding for the very first time. I used a pedal wrench and tightened it. The pedal was indicated as being the "left" pedal and has reverse threads. Now the threads on the left crank are about half stripped. I have successfully screwed the pedal on but am worried about it failing now. The crankset is a Shimano FC-RS500. I would post a picture but I don't want to do anymore damage to the threads by taking it off again. Do I replace the left crank now or just wait and see?

I'd be contacting whereever you got the bike from and raise the question of warranty - given its a brand new bike.

Warranty may be void if you attempt to repair it yourself.

Also, check whether self-assembly voids the warranty... some have a rider that "bike must be assembled by an authorised dealer" I think Giant has this requirement.