Drill America DWT64017 4"-8 NPT HSS 8 Flute Pipe Tap, Qualtech, DWT64017Drill America DWT64017 4" 8 NPT HSS 8 Flute Pipe Tap, Qualtech, DWT64017 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