Thread Seal

Pipe and pipe components such as fittings are typically joined together (fitted) by threads which are cut on the OD (outside diameter) of the pipe.

Pipe threads are of two basic types, namely

  • Tapered Threads
  • Straight (or Parallel) Threads

The Americans call the latter "straight," whereas the British call them "parallel." For all intents and purposes, Straight threads are Parallel, and Parallel threads are Straight.

Tapered Threads

This is the default type of thread on a pipe. If it's a "pipe thread," it's most likely a tapered pipe thread, unless specified otherwise.

The tapered thread was an ingenious invention: they engage easily, go in a number of turns, and then bind (the male thread "expanding" out against the female thread) so as to create a fluid-tight seal.

Traditionally, tapered threads require a sealing medium such as "pipe dope" in order to actually become fluid-tight. More modern systems, such as the "dryseal" thread (see NPTF) involve very tight manufacturing tolerances such that the male and female threads fit together so closely that no sealant is required.

I don't think I'm alone, however, in that I don't buy it—I lack the faith in dryseal threads to believe that they will not leak. Therefore, I almost always use a sealant, regardless of whether the thread is NPTF or just NPT. Half the time you never really know whether both fittings are NPTF anyway, so why not just add some sealant and be done with it? Who needs the headache of a leak, when it's so easy to avoid? —$.02

There are, however, cases where the use of a sealant should be avoided, due to incompatibility with the working fluid, and the subsequent contamination issues that might result from its use. These cases are typically found in the manufacture of new systems or devices, where careful control of all components assures thread compatibility, etc. But if I'm in the field, replacing a fuel pump on a Crummy, I'm gonna dope the thing up, that's all.

Straight (Parallel) Threads

Straight or parallel threads do not seal on the thread. Therefore, these threads require an additional accessory in order to effect a seal. Said accessory is usually an o-ring or a gasket. The most familiar example is a garden hose, with which we are all familiar. Who hasn't experienced the old, dried-up, cracked and malfunctioned hose gasket? The water just runs down your wrist, or sprays out onto your arms and legs. This experience teaches us that straight threads don't seal.

We define two basic configurations of straight thread sealing strategies, namely

  • Base Seat Thread
  • Tip Seat Thread

These types are referenced to the male fitment. In other words, the seal is affected either at the base of the male threads, or at the tip of the male threads. The garden hose is an example of a Tip Seat Thread fitment. An example of a Base Seat Thread fitment is the ORB (O-Ring Boss) which is an SAE specification.

 

The Street Elbow... where did it get its name?

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.

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

---

 

 

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.