Threaded pipe

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A threaded pipe plug; note that the threaded portion is tapered and so touches the calipers at only two points.

A threaded pipe is a pipe with screw-threaded ends for assembly. Threads are generally slightly conical rather than cylindrical (like a bolt or power screw). As a result a threaded pipe joint is much like a conical plug plugging a conical hole. In addition, the threads act as a labyrinth seal. While no gasket is needed, a coating is usually applied to the threads such as teflon tape.

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.

Due to the taper (cone) of the thread, when such threads are cut by a tap or a die, increasingly more material is being cut at a time as the length of the thread is increased. Beyond a pipe sizes of a fraction of an inch, it is therefore common to use powered tools when cutting such threads.

Threaded fittings are sometimes used on plastic piping. Due to the wedging effect of the tapered thread, extreme care must be used to avoid overstressing the female component of the joint. It is not uncommon for such fittings to split days, weeks, or even years after initial installation, and therefore many municipal plumbing codes restrict the use of threaded plastic pipe fittings.