My fork bends when doing wheelies! 
How can i fix this?
So i bought a 1980's schwinn world sport, great condition, practically new. Converted it to a fixed gear, and when i try to do wheelies and come back down, it usually slams pretty hard, and the fork or bearings or something pops out of place or gets bent, and it makes the handlebars get stuck when i turn. I can bend it back, but i'd rather find a solution to the problem. The stem/fork is unthreaded, and original. Would a threaded fork help? or is this unavoidable and i just need to set it down slowly? can i even put a threaded fork on a frame like this?
How can you fix this? Simple: Don't pop wheelies, jump curbs, etc. That bike was never designed or built to handle that kind of abuse.