Threaded Pipe - Tapered Threads

Tapered Threads

The threaded pipes used in some plumbing installations for the delivery of 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 provided by a threaded pipe joint depends upon the labyrinth seal created by the threads; upon a positive seal between the threads created by the deformation of the threads when they are tightened to the proper torque; and sometimes on the presence of a sealing coating, such as thread seal tape ("Teflon tape"), or a liquid or paste pipe sealant such as pipe dope. Tapered thread joints typically do not include a gasket.

Especially precise threads are known as "dry fit" or "dry seal" meaning that no sealant is required for a gas-tight seal. Such threads are needed where the sealant would contaminate or react with the media inside the piping, e.g., oxygen service.

Tapered threaded fittings are sometimes used on plastic piping. Due to the wedging effect of the tapered thread, extreme care must be used to avoid overtightening the joint. The overstressed female fitting may split days, weeks, or even years after initial installation. Therefore many municipal plumbing codes restrict the use of threaded plastic pipe fittings.