Unfortunately, this is a common assembly error. To avoid it, the left pedal and crank arm must be installed on the left hand side. The left pedal is reverse thread (turn counter clockwise by hand to install). The right pedal and right crank arm must be installed on the right side (normal thread, turn clockwise by hand to install) .Begin threading the pedals by hand, once screwing in smoothly by hand, then you can tighten with a wrench.
Often if installed improperly, the crank arm will need to be replaced. Typically, the pedals are ok, because they have steel threads, where as the crank arm is aluminum and the threads are softer and often damaged beyond repair. Depending on the bike & which side is stripped, you may be able to replace just one. For non-drive side cranks (L) You can order single replacements (make sure it matches your other crank, diamond style or square shown below). The drive side (R)/ Side with chainring, generally come in a set of both Left + Right cranks.
If replacing left or right crank, PLEASE VERIFY, is yours SQUARE or DIAMOND. The new part needs to be the same as the one you have. Or alternatively,both can be replaced, but can be more expensive and you will need a crank puller tool (about $10.00).
500 Series, 700 Series, LMT'D & Roadster Ghost left crank arm diamond:
500 Series, 700 Series & Roadster Ghost Replacement Crankset:
Full Crankset (must swap over existing chainring)
If you have additional questions, please contact us and include info on which bike you have & a photo of the crank arm.