in 16 inches of length () measured by the change of diameter (of the pipe thread) over distance of thread. The angle between the taper and the center axis of the pipe is tan−1() ≈ 1.7833° ≈ 1° 47′. The standard for Nominal Pipe Size (often abbreviated NPS, which should not be confused with the abbreviation NPS for the straight thread form standard) is loosely related to the inside diameter of Schedule 40 series of sizes. Because of the pipe wall thickness of Schedule pipe, the actual diameter of the NPT threads is larger than the Nominal Pipe Size outside diameter, and considerably so for small sizes. Pipe of a given size in a different Schedule than Schedule 40 provides a different wall thickness while maintaining the same outside diameter and thread profile as Schedule 40. Thus the inside diameter of a given size of Schedule pipe differs from the Nominal Pipe Size, while outside diameters for a given nominal Schedule size are the same between Schedules. Thread form National Pipe Thread series have the same 60° included angle as Sellers thread. On the other hand, for sealing capability, tapered (pressure-tight) threads have crest and root shapes that differ from Sellers. For example, the NPT has a crest width of 0.038 to 0.062 pitch, which is sharper than the 0.125 pitch of Sellers. National pipe taper fuel (NPTF) National Pipe Taper Fuel (NPTF, also called Dryseal American National Standard Taper Pipe Thread, defined by ASME B1.20.3) is designed to provide a more leak-free seal without the use of PTFE tape (often referred to by the popular brand name "Teflon") or another sealant compound. NPTF threads have the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path. Standard sizes History In 1864, William Sellers, then president of Franklin Institute, presented a standard for nuts, bolts, and screws. Out of this effort came the first United States Standard threads, including pipe threads. See also AN thread British Standard Pipe Fire hose thread Garden hose thread Gender of connectors and fasteners Panzergewinde Thread angle Threaded pipe References Bibliography External links NPT Pipe Thread Dimensions ASME B1.20.-1983 Pipe Threads, General Purpose, Inch NPT Taper Pipe Threads Thread standards Piping Plumbing Standards of the United States ru:Трубная резьба