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NPT vs. BSP Pipe

While NPT threads are common in the United States, BSP threads are widely used in many other countries.  I have found that my Japanese-built injection mold presses have predominantly BSP fittings. 

American National Pipe - NPT/NPS

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.

British Standard Pipe - BSP

BSP: British Standard Pipe

BSP pipe, Like American National pipe (NPT, NPSM), is designated by trade size, rather than actual diameter, which is approximately equal to the thread's Major Diameter in the table below.

There are two types of BSP threads:

  • BSPT: British Standard Pipe Taper -also known as "R" or "Rc" threads
  • BSPP: British Standard Pipe Parallel -also known as "G" or "Rp" threads

Both styles have the same thread angle, shape, and pitch (threads per inch).  However, BSPT threads are tapered and BSPP threads are straight (parallel).  BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).

Here are the actual thread dimension data for BSPP and BSPT threads.  The major diameter is a bit larger smaller than the actual OD of the pipe, and the minor diameter should be very close to what you would measure inside the female threaded end of a fitting.  Note that the Gage Length dimension pertains only to the BSPT (tapered) thread. 

Thread Type Compatibility

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.

 ** We had previously indicated GHT as compatible with 3/4-NH.  This was incorrect.  Oops... sorry.

Flare Fittings

The following flare fittings are similar, but not equivalent:

  • AC (Air Corp Standard) also called Parker Triple Fitting (NOT Triple-Lok)
  • AN (Air Force - Navy Aeronautical Standard)
  • AND (Air Force - Navy Aeronautical Design Standard)
  • AS (Aerospace Standard)
  • MS (Military Standard)
  • JIC (Joint Industry Council)
  • SAE 37° also called Parker Triple-Lok
  • ISO 8434 (1986), ISO 8434-2 (1996)
  • SAE 45°

AC (Air Corp)

The AC flare fittings were developed for the aviation industry in the 1930s, and have long since been superceded by AN.  Today, it is rare that you will come across one of these fittings.  AC fittings use a 30° flare angle, and they are most readily recognizable by a very short or non-existent straight section between the top of the thread (on the male fitting) and the major diameter of the flare.  Most flare fittings of the other types have a non-threaded section at the base of the flare.

Note that the AC were talking about here has nothing to do with air conditioning.  It is possible that some people may use the term "AC Flare" to refer to the SAE 45° fittings discussed below.

AN, AND, MS, AS

AN / AND fittings have a 37° flare angle and use a class 3A/3B thread, which is a close tolerance, radiused root thread.  These fittings have traditionally been available in aluminum, carbon steel, stainless steel, and titanium.

AN / AND fittings remain in prevalent use today, and they may alternatively be designated with an MS number, which are generally directly cross-referenced replacement fittings (functionally equivalent).  For instance, an AN815 is equivalent to an MS24392.  MS, however, is a very broad standard covering all kinds of hardware, and is not limited to AN fluid couplings.

The AN and MS designations for many fittings are being superceded with AS (Aerospace Standard).

In general, there appears to be a trend away from flared tubing in favor of compression type fittings, probably due to the fact that flares require greater special attention to craftsmanship (and more time) in assembly

JIC, SAE 37°, ISO 8434, Parker Triple-Lok

When someone says it's a JIC fitting, this is generally what they are talking about.  This fitting is the general industrial adaptation of the AN fitting.  These fittings have a 37° flare angle and use a class 2A/2B thread, which is a standard tolerance industrial thread with normal trapezoidal root truncation.  These fittings have traditionally been available in carbon steel, stainless steel, and brass. Some of these fittings may conform to MIL-F-18866 and/or SAE-J514.

SAE 45°

The SAE 45° fitting is typically used for low pressure applications, usually with soft copper or soft aluminum tubing, which flares easily to the 45° angle.  These fittings are easily recognized by placing them on a square, or by placing two males flare-against-flare at right angles, and observing the 90 degree included flare angle,  The 45° flare fitting is the one you are most likely to run across at your local hardware store.  They are commonly found in use in the following applications:

  • refrigerant lines
  • fuel lines
  • natural gas appliance hookups
  • ice maker connections
  • domestic water systems in motor homes

JIC 37° / SAE 45° Dual Angle Seat

There are some fittings made which can be screwed onto either a 37° or a 45° fitting.  The dual angle flare seat is apparent.  Obviously, this is a kind of bastard fitting, and should not be selected as a primary style in the design of any hydraulic system, though they surely come in handy at times.

 
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
---