What is a Tapered Thread?

Threaded pipe fittings for use in installations for the delivery of air, gases or fluids under pressure have a threaded section that is slightly conical (in contrast to the cylindrical section commonly found on bolts and leadscrews). This is called a "tapered thread". The seal attained within a threaded pipe joint depends upon the seal created by the deformation of the threads when they are tightened to the proper torque and, in almost all applications, the use of a sealing coating such as thread seal tape ("Teflon tape"), a liquid or paste pipe sealant such as pipe dope, or pre-applied sealant. Tapered thread joints typically do not include a gasket.

"Visual" of a Tapered Thread pipe plug; note that the threaded portion is tapered and so touches the calipers at only two points. Photo by Ben Frantz Dale