Threads

BSP

BSP thread form stands for British Standard Pipe and is common in Australia and the commonwealth countries. It is based on trade size rather than actual diameter which can lead to some confusion when measuring ports. 

There are two types of BSP threads;
- Parrallel (BSPP) - also known as G or Rp
- Tapered (BSPT) - also know as R or Rc

Both threads have the same pitch, angle (55 degrees) and shape (rounded peaks and valleys).

The below table gives the major and minor diameter for each BSP Trade Thread Size.

Trade Size Threads per inch Pitch Major Diameter Minor Diameter Gage Length
Inch mm Inch mm Inch Mm Inch mm
1/8 28 0.0357 0.907 0.383 9.728 0.3372 8.565 0.1563 3.97
1/4 19 0.0526 1.337 0.518 13.157 0.4506 11.445 0.2367 6.012
3/8 19 0.0526 1.337 0.656 16.662 0.5886 14.95 0.25 6.35
1/2 14 0.0714 1.814 0.825 20.955 0.7336 18.633 0.3214 8.164
3/4 14 0.0714 1.814 1.041 26.441 0.9496 24.12 0.375 9.525
1 11 0.0909 2.309 1.309 33.249 1.1926 30.292 0.4091 10.391
1 ¼ 11 0.0909 2.309 1.65 41.91 1.5336 38.953 0.5 12.7
1 ½ 11 0.0909 2.309 1.882 47.803 1.7656 44.846 0.5 12.7
2 11 0.0909 2.309 2.347 59.614 2.2306 56.657 0.625 15.875
2 ½ 11 0.0909 2.309 2.96 75.184 2.8436 72.227 0.6875 17.463
3 11 0.0909 2.309 3.46 87.884 3.3436 84.927 0.8125 20.638
4 11 0.0909 2.309 4.45 113.03 4.3336 110.073    


​NPT

NPT stands for National Pipe Thread and is an American standard thread. It may also be reffered to as MPT , MNPT or NPT (M) for male external threads and FPT, FNPT or NPT(F) for female interal threads. A thread sealant must always be used to achieve a leak free seal (except for NPTF). It is also based on Trade Size rather than actual diameter (similar to BSP in this regard).

Both threads have the same pitch, angle (60 degrees) and shape (flat peaks and valleys).

The below table gives the Threads Per Inch, Pithc and Major Diameter for NPT Threads.

Trade Size Threads per inch Pitch Major Diameter (O.D)
Inch mm Inch mm
1/8 27 0.03704 0.94082 0.405 10.29
1/4 18 0.05556 1.41122 0.54 13.72
3/8 18 0.05556 1.41122 0.675 17.15
1/2 14 0.07143 1.81432 0.84 21.34
3/4 14 0.07143 1.81432 1.05 26.67
1 11 ½ 0.08696 2.20878 1.315 33.4
1 ¼ 11 ½ 0.08696 2.20878 1.66 42.16
1 ½ 11 ½ 0.08696 2.20878 1.9 48.26
2 11 ½ 0.08696 2.20878 2.375 60.33
2 ½ 8 0.125 3.175 2.875 73.03
3 8 0.125 3.175 3.5 88.9
4 8 0.125 3.175 4.5 114.3


BSP vs NPT

NPT threads are common in the United States and a few other countries, BSP threads are widely used in many other countries.

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 equivalent is NPT and BSPP’s equivalent is NPS.

Never swap threads if it is a high pressure application.

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

NPT and BSP thread pitches (threads per inch) are listed below. To determine pitch, use a thread gauge or count the number of threads that fall into a 1" span.

Trade Size Pitch (Threads per Inch)
NPT/NPS BSP
1/8 27 28
1/4 18 19
3/8 18 19
1/2 14 14
¾ 14 14
1 11 ½ 11
1 ¼ 11 ½ 11
1 ½ 11 ½ 11
2 11 ½ 11
2 ½ 8 11
3 8 11
3 ½ 8 11
4 8 11
5 8 11
6 8 11