Originally Posted by
HillRider
Absolutely, take that bike back to the dealer and have it repaired properly and that probably means having the crank replaced. There is no reason for a new bike sold by any decent bike shop to have that happen as the dealer should have gone over the bike for correct set up before you ever bought it.
This. Riding with a loose crankarm may have damaged it, so the shop needs to deal with it. Totally a warranty issue. Don't address it yourself.
FWIW, these things happen, even with good shops. My shop recently put (back) together one of my bikes after having the frame repainted, and the crankarm came loose after about 1,000 miles...When I took it in, the mechanic apologized profusely, and told me that, some time after working on the bike, he had realized that his torque wrench was malfunctioning. They replaced the crankarm and the pre-load cap, used loctite on everything, etc. Stuff happens.