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What is a Pipe Thread

By STG Machine

Threads on pipes and fittings are commonly referred to as pipe threads. These types of threads will create a pressure tight joint when properly assembled. There are many different types of pipe threads and some industries even have their own definitions for them. Rather than giving a full breakdown of all of the different types of pipe fittings, this article will focus on the most common ones.

Common Types of Pipe Threads:

  1. NPT: American National Standard Taper
  2. NPTR: American National Standard Taper for Railing Joints
  3. NPSC: American National Standard Straight for Couplings
  4. NPSM: American National Standard Straight for free-fitting mechanical joints
  5. NPSL: American National Standard Straight for Loose fitting mechanical joints with locknuts
  6. NPSH: American National Standard Straight for hose couplings

Overview of Each

NPT:  This is the most common of the pipe thread family. The thread angle is 60 degrees and further information such as pitch,TPI and pitch diameter at the beginning of  a thread can be found online or in manufacturing tables. The taper of a NPT thread is .75 inches per foot or 1 degree. There are additional tables available on the engagement between external and internal threads when screwed together. 

NPTR: These threads are slightly modified from more traditional NPT threads. The ½ through 2 inch sizes are shortened by 3 threads and the 2.5-4 inch sizes are shortened by 4 threads. 

NPSC: These threads are the same as NPT threads but the threads are straight and parallel to each other. Under low pressure they can be utilized to form a pressure tight seal.

NPSM, NPSL and NPSH: These threads are all based off of special thread tables where the external and internal threads slightly differ compared to NPT threads. They are typically more loosely connected, hence their name and the difference in major and minor diameters.

We would be remiss without mentioning that there are many other types of pipe threads available. Some examples include: Dryseal, British standard, Fire hose connections, interference-fit, spark plugs and Swiss Screws.

Additional manufacturing information will be published in the future. However, we do have an article on basic thread manufacturing principles here. Feel free to contact us for prototype or custom pipe threads, but online manufacturers such as Mcmaster or Home-Depot can be useful too.

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