Originally Posted by
JohnDThompson
As mentioned above, if it has been ridden while loose, chances are the square broach in the arm has been damaged. You can try fixing it, as [MENTION=410984]Trevtassie[/MENTION] suggests above, but in my experience this is seldom successful. A replacement non-drive side arm shouldn't be too costly, though. Unless you're a stickler for looks, it doesn't have to exactly match the old arm. Just try to get the same length as the original.
Also, there's "squares" and there's "diamonds". Be sure that the one you buy matches the one you're replacing. Otherwise your son is going to develop a real funny peddling style.
30 lb/ft. If you don't have a torque wrench imagine a 30 lb. weight hanging on the end of a foot long wrench. If you're using a 6 in Allen key, you probably aren't getting 30 lb/ft and you may get to do the job again. Too much torque, by the way, is bad too. It will crack your new crank arm at one of the corners of the square hole.