What if it keeps coming loose after tightening the bolt. What would be wrong

 

If it has been running loose, at least replace the pulley so that the hole is the right size. And hope the crank isn't too messed up.

Bolt tightness spec is 140 ft-lb. Or tighten to 14 ft lb then put an impact on and turn 1/4 turn additional. The latter method might not work properly if the parts are torn up already.

 

Clean the crank and bolt threads really well. Use red loctite and let it set up before driving it.

 

Add required car and engine information, and give some background.

 

Loctite. Um, I would use blue.