I took off my rear derailleur to clean it, and when I went to put it back on the threads were too damaged to get it back on. I've never had this happen before; anyone else? Can I get it re-tapped? Will it be as good as new? I'm nervous that it's been damaged by road salt, and lost metal. It's a nice steel bike that's only about three years old, but I've ridden it every day through Minnesota winters.