Why Aren tapered pipe threads (NPT) available in swivels?
They typical coupling consists of three main components; male, band, swivel. The swivel’s job is to bring together the male and the band, squeezing a gasket between them, sealing the water path. With straight threads, like NH and NPSH, the swivel turns on the male as far as necessary to make this seal. However, NPT threaded components seal by jamming the mating threads as they are tightened. No gasket is used. The turns required to tighten may vary. If NPT threads are used in a coupling, the end of the male may not stop at the correct place in the swivel. The male can be tight into the swivel but, even with a gasket, it may not seal against the band. This would allow water to leak out of the joint. Thus, NPT threads are not available in swivels. However, a coupling set with a NPT male and a NPSH swivel can work. NPSH thread is the straight version of NPT thread. A NPT male will screw into a gasketed NPSH swivel and seal against the band, provided the end of the male is cut square.