I'm working on a converting an older road bike to a fixed gear. The bottom bracket is in pretty good shape, but has a threaded spindle stud, so there's a nut that screws on to the spindle to hold the crank arms in place. On the drive side, everything worked out when I installed a single-speed crank that I had lying around, the chainline is even good.

On the left side, the recess in the crank arm doesn't leave enough space to screw the nut onto the spindle. I don't know why one crank arm fits and the other doesn't. I'm looking to buy a new crankset, anyway, but I want to make sure I get something that will fit on the spindle, since I'd like to keep the bottom bracket.

Any advice?