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In order for two components to fit properly, thread types must be compatible. See the list below for thread types that can be used together.

Thread Compatibility

 

Over the ages, pipe has evolved from the ceramic and stone pipes of the Roman empire and before, to our modern version, the iron pipe.  There are today, of course, countless variations, many of which we hope to explore and explain herein.  Our treatment of the subject is by no means an authoritative development of the history of pipe development.

For our purposes the most basic pipe is good old fashioned Iron Pipe.

"Iron Pipe" is a bit of a figure of speech, used because it describes adequately the dimensional characteristics of what we're talking about. That being said, iron pipe (or just plain pipe) can be made out of steel of various grades, stainless steel, and often times brass or bronze, and sometimes copper or plastic.  

Be careful about copper and plastic pipe though, because these materials are also broadly used in the manufacture of tubing which (dimensionally speaking) is not pipe.  Adding further to the confusion, copper pipe (as opposed to copper tubing) is usually called pipe and, like iron pipe, is size-designated by a "nominal" (or trade size) rather than actual diameter, but using different actual diameters from those of iron pipe. For a fuller explanation, please refer to our Abstract: About Copper Pipe (coming soon).

British vs. American Pipe

As for Iron Pipe, it appears that the world has gone down two roads on pipe standards: The British, and The American. Someone might ask, "Why did the Americans feel the need to muck things up? Why couldn't they just go with the flow and use the British standard like everyone else? This is a good question, to which I have my own answer, based on my personal suspicions and speculations. This is not authoritative, it's just my hunch.

One thing everyone must agree on is that the Americans have historically been pretty good at finding the most economical way of making something that works. It may not be the most elegant, or most excellent, but "it's low-cost, it gets the job done, and it works." This brings us to the crux of the break between British pipe and American pipe... it's the threads (or more particularly, the thread forms)!

Going way back, the British standard for thread-making is the Whitworth thread, which was defined by Joseph Whitwoth. The Whitworth thread is a beautiful thing in its own right. Thing is, it's difficult to cut. So back in the early days of the industrial revolution, an American engineer named William Sellers defined and promoted a standard thread form which was subsequently adopted by all of American industry. And as it turns out, it was a very good standard, and it made thread cutting of matched male- and female-threaded parts much easier than was the case for Whitworth threads. The adoption of this standard was surely helpful to the industry, even though it had the potential of introducing myriad thread compatibility issues with zillions of already-existing hardware items in other places.

But think for a moment about this incompatibility issue... If an American coach builder in the late 1800's had the choice of importing his nuts and bolts from England or buying them from the machine shop in Milwaukee (or even just down the street), which would he choose? Today we can order stuff from China on ebay and it gets here in a few days. But back then importing foreign goods was a whole 'nother issue! It took months. So if we Americans cut different threads to those of the Brits, who cares? I mean, realistically, what difference did it make? It wasn't as though the land was already filled with Range Rovers and Fiats. Compatibility simply wasn't much of an issue. Economics, and cost of manufacture, however, were very real issues.

American National pipe (NPT, NPS), Like British Standard Pipe (BSP), is designated by trade size, rather than actual diameter, as shown in the table below.

There are two basic types of National pipe threads:

  • NPT: National Pipe Taper
  • NPS: National Pipe Straight

NPT threads are also sometimes referred to as

  • MIP (Male Iron Pipe)
  • FIP (Female Iron Pipe)
  • IPT (Iron Pipe Thread)
  • FPT (Female Pipe Thread)
  • MPT (Male Pipe Thread)

Note that these references are somewhat casual, and might possibly be used in reference to NPS instead of NPT.

Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch).  However, NPT threads are tapered and NPS threads are straight (parallel).  Both threads have a 60° included angle and have flat peaks and valleys (this is a Sellers thread form).

If you've worked with pipe much at all, you've probably noticed that the size of the pipe isn't really what size the pipe is.  Unlike tubing, which is generally specified by its OD, or hose, which is generally specified by its ID, pipe is specified by something else... its Trade Size.  So when you say "3/4 pipe," you're actually saying "pipe whose OD is a little more than an inch, and whose ID is about 53/64."  -that is, if you are talking about schedule 40 pipe, which is generally what is used for most plumbing applications.

Pipe dimensions are specified by trade size and schedule, according to the following table.  Note that while British Standard Pipe dimensions are similar, they are not equivalent to the American Standard Pipe Sizes.  See NPT vs. BSP Pipe for thread comparisons.

While NPT/NPS threads are common in the United States and Canada, BSPT/BSPP (collectively, BSP) threads are widely used in many other countries. I have found that my Japanese-built injection mold presses have predominantly BSP fittings. 

  • BSPT -British Standard Pipe Taper
  • BSPP -British Standard Pipe Parallel
  • NPT -National Pipe Taper
  • NPS -National Pipe Straight

While the actual specified outside diameters of American National Pipe differ slightly from those of British Standard Pipe, either thread may reliably be cut onto a pipe of its respective trade size.  BSPT and BSPP threads are analogous to NPT  and NPS threads, respectively.

WARNING:  Never, never try to mate a BSP fitting with an NPT or NPS fitting if the pressure holding capability is at all critical.

NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. NPT/NPS threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).

NPT and BSP thread pitches (threads per inch, TPI) are listed below. To determine pitch, use a thread gauge or count the number of threads that fall into a 1" span.  Note that, strictly speaking, when we use threads per inch, we are actually specifying the inverse of the pitch, pitch being in units of [length] / [peak to peak].  Metric threads are usually specified in actual pitch, e.g., 1.5mm, 2.0mm, etc.  This is the actual length of each thread, peak to peak.  Although the term "pitch" is universally used, albeit loosely, to describe threads per inch, the actual pitch of a 1/4BSP fitting is really 1/19 inch, or 0.0526 inches.

 

 

 

Pipe Size

Pitch  (Threads/Inch)

NPT/NPS

BSP

 1/16"

27

---

 1/8"

27

28

 1/4"

18

19

 3/8"

18

19

 1/2"

14

14

 5/8"

---

14

 

Pipe Size

Pitch  (Threads/Inch)

NPT/NPS

BSP

 3/4"

14

14

1"

11 1/2

11

1 1/4"

11 1/2

11

1 1/2"

11 1/2

11

2"

11 1/2

11

2 1/2"

8

11

 

Pipe Size

Pitch  (Threads/Inch)

NPT/NPS

BSP

3"

8

11

3 1/2"

8

11

4"

8

11

5"

8

11

6"

8

11

8"

8

---

 

 

Iron Pipe► Fitment

This thread is used on stair banister railings and similar mechanical applications.  Care must be taken not to confuse this thr

NPTR Data Table alt

NST (National Standard Thread) is a definition of actual diametral thread dimensions and pitches for various fittings for adapting nominal hose and pipe sizes.

BSPT: British Standard Pipe Taper -also known as "R" or "Rc" threads

American National Pipe Taper (NPT), Like British Standard Pipe (BSP), is designated by trade size, rather than actual diameter, as shown in the NPT/NPS Table.

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