Taper taps have eight to ten threads chamfered to start and cut threads easily, in a through hole, which is open at the bottom.
Plug taps have three to five threads chamfered, to cut threads in a through hole, or to start threads in a blind hole, closed at the bottom.
Tap Drills are smaller in diameter than the screw shaft to leave enough metal to cut threads.
Clearance Drills are large enough to allow the body of the bolt to pass through the hole.
Tap Drill: for 6-40 thread, use No. Drill-33
Clearance Drill: for 6-40 thread, use No. Drill-25
Beware of HSS hardware store taps, and machine shop taps. Never use high speed steel (HSS) taps for gunsmithing. High speed steel makes a great drill bit, but is a poor choice for a gunsmith's tap. If broken, it is difficult to remove. Our high carbon steel taps are hard and sharp. If broken, our taps are easily removed with a drift punch.