2"-11-1/2 NPT HSS Interrupted Thread Pipe Tap. National Pipe Taper Taps (NPT) 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 chamger. NPT threads require the use of a "sealer" like teflon tape or pipe compound.