I suspect that what you're thinking is thread damage extending all the way across the threads is actually the weld seam on the bottom bracket shell. That type of shell is bulge-formed from sheet metal to create the sockets for the tubes, then wrapped into a cylinder and welded closed before cutting the threads. It's likely that all you need to do is chase the threads, as others have suggested. Grease on the threads will also help. If you still have problems, seek a bike shop that has professional-grade, piloted bottom bracket taps to get the job done right.