These professional thread-restoring tools are designed to not undercut or damage existing threads. The comprehensive 40-piece kit includes taps and dies for national coarse (silver), s.a.e. national fine (black), and metric (gold). Two thread files are also included to dress both s.a.e. and metric threads. This set contains: nc 1/4" x 20 to 5/8" x 11 dies and 1/4" x 20 to 9/16" x 12 taps; nf 1/4" x 28 to 5/8" x 18 dies and 1/4" x 28 to 9/16" x 18 taps; and metric 6 x 1.000 to 12 x 1.75 taps and dies. Tools are simply turned with your socket wrench or open end wrench.