What you need is a thread file. I have 3 of them. Hayes File Company makes them. No. 1 20 24 13 14 16 18 11 12 thread/inch No. 2 28 32 9 10 20 27 12 16 thread/inch also have a metric by K & D Very handy tools. If you don't have access to a thread file, as someone mentioned a small rat tail file can be used or a hacksaw blade.