Also called 'pipe taps', National pipe taper taps are appropriate for Threading a wide variety of materials both ferrous and non-ferrous. The nominal size of a pipe Tap is that of the pipe fitting to be tapped, not the actual size of the Tap. The thread tapers 3/4 of an inch per foot. All pipe taps furnished with a 2-1/2 to 3-1/2 thread chamfer. Npt threads require the use of a “sealer” like Teflon tape or pipe compound.