I thought I would pass this along, since I searched the archives before I tried it, and didn't find any reference to fixing or straightening bent crank arms.
I am in the process of restoring an older (1957) English bike with a cottered Nicklin chainset. The drive-side arm was bent in, towards the frame. I removed and cleaned the chainset, and took the pedals off the arms.
To straighten the drive side arm, I put it in a 400 degree (F) oven for five minutes, then with oven mitts ran it up to my shop, put the pedal end in the bench vice with the arm pointing straight up and the chainwheel at the top. I then slipped the handle from a socket wrench into the hole for the BB spindle, and pushed down. I had to put my whole weight on the thing. However, the result was that it actually bent back very close to straight.
Now there are about a dozen things I can think of off-hand which could have gone wrong, so I don't recommend this unless you're willing to throw out the crank if it doesn't work. But mine came out beautifully, and the chrome looks as great as ever.
Again, thought I'd pass this along in case someone like me goes searching the archives some day for a tip on how to straighten a bent crank arm.
Oh yes, and of course you should remember, the straightened arm may never be as stiff as it was originally, so no out-of-the-saddle sprints on it, OK?
Duncan Granger
Lancaster, PA