Types of threads (II): NPT threads

04 December 2015 / By: rmmcia

In a previous post we talked about BSP parallel threads (cylindrical) or “Gas” threads (G), the ones rmmcia produces at the standard brass fittings catalogue for threaded tube and pipe, according to regulation ISO 228-1.

This type of thread Whitworth or BSP, became the British standard from mid XIX century. A couple of decades later the NPT thread (National Pipe Thread) was developed in the United States. William Sellers, an important mechanic engineer and businessman, designed it and showed it in 1864 during a meeting as member of the Franklin Institute. In a few years it was adopted by the main railway companies and it was used in all state public works, taking the name of Sellers Thread or Franklin Institute Systems. Now it is the most used type of thread in the United States and also Canada.

npt ansi b1.20.1 npt nptf threads

They differentiate from BSP threads in the thread angle, which is of 60 degrees instead of 55, and edges are pronounced. There are also differences in the thread pitch, at some of the sizes. For instance, at BSP one inch thread, the thread pitch is of 11 threads per inch, whereas at NPT one inch, the thread pitch is of 11’5 threads per inch.

NPT threads are tapered (1º 47’ taper angle), though there are also some versions in parallel format.

  • NPT (National Pipe Thread): naming of this thread consists of initials NPT, diameter – nominal diameter- and thread pitch – put into words as threads per inch. NPT specifications are collected at regulation ANSI B1.20.1, which substituted the original ANSI B2.1 in 1973.
    NPT thread requires the use of a sealant product, such as a PTFE tape.

          Example: 3/8 - 18 NPT indicates NPT thread, for a 3/8” pipe and thread pitch
          of 18 threads per inch.

  • NPTF (Dryseal Pipe Threads, “f” refers to fuel): this thread derivates from NPT. It shares with NPT the same diameter, threads per inch and taper shape. It differentiates from NPT in roots and crests’ size, making them interfere with each other at the tightening when assembling. This way it ensures correct sealing, without the need of sealants. 
    Naming of this thread consists of initials NPTF, nominal diameter, thread pitch and class (1 or 2). NPTF specifications are collected at regulation ANSI B20.3, which substituted the original ANSI B2.2 in 1973.
    Calibre’s specifications for verifying NPTF threads are collected at regulation ANSI B.20.5.

          Example: 1/8 - 27 NPTF-1, indicates NPTF thread, 3/8” diameter and thread pitch
          of 27 threads per inch, and class 1.

          npt ansi b1.20.1

          ANSI B1.20.1 Taper thread
          60º thread angle
          1º47” taper angle

We find other North American standards, such as:

  • NPSC American Standard Straight Coupling Pipe Thread
  • NPTR American Standard Taper Railing Pipe Thread
  • NPSM American Standard Straight Mechanical Pipe Thread
  • NPSL American Standard Straight Locknut Pipe Thread

At rmmcia we manufacture also taper threads upon request, as per ISO 7-1 or other as the american NPT (National Pipe Thread). If you need a customized making following this specifications, don’t hesitate to contact us.
For more technical information feel free to check with us or refer directly to regulations.

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