you need to have a flip flop hub which allows you to run various sizes. one side runs 14 /15 tooth, and some 13 tooth freewheels, and the other side allows you to run 16/17/18 tooth freewheels. if you want to run a 13 you'll need that sized thread, stripping the hubs wont help, you need to change the hub. if you want to run all sizes of sprocket size get a cassete hub, were you can run as low as a 9 tooth up to 18. hope that helps.
alan