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BS 21 - Pressure Tight Pipe Thread Joints

BSPT - British Standard Pipe Taper threads & BSP or BSPF - British Standard Pipe Parallel (Straight) threads.

Threads according BSi - British Standard Institute - BS 21 "Specification for pipe threads for tubes and fittings where pressure-tight joints are made on the threads" can be made as

  • parallel (straight) threads
  • taper threads

Note that the BS 21 is withdrawn, superseded and replaced by BS EN 10226-1:2004, BS EN 10226-2:2005, BS EN 10226-3:2005. Product threads in ISO 7 and BS 21 and EN 10226 are identical and fully interchangeable.

Parallel (straight) Threads

BS parallel threads are also referred to as British Gas, British Pipe Parallel or Parallel Fastening Thread.

Common symbols used for the thread: BSP, BSPP, BSSPI, BSPF, BSPG, PS, Rp, G

The BSPP (parallel) male will mate with a BSPP (parallel) female or a female port.

Usually marked: G size ISO

Taper Threads

BS taper threads are also referred to as British Standard Taper Pipe, Pipe Taper or Conical Thread.

Common symbols used for the thread: BSPT, BSPTr, PT, KR, R, Rc

Usually marked: Rc size ISO

Characteristics of BSPT taper threads:

  • tapered thread 1o47'
  • truncation of roots and crests are rounded
  • 55o thread angle
  • pitch is measured in threads per inch
  • taper is 3/4" per foot or 1 in 16 on the diameter
BS 21 - Pressure Tight Pipe Thread Joints
Pipe Size
(inches)
Threads per Inch
TPI - Pitch
Major Diameter
OD
(inches)
Pitch
(mm)
Major Diameter
(mm)
Minor Diameter
(mm)
1/16" 28 0.304 0.907 7.723 6.561
1/8" 28 0.383 0.907 9.728 8.566
1/4" 19 0.518 1.337 13.157 11.445
3/8" 19 0.656 1.337 16.662 14.950
1/2" 14 0.825 1.814 20.955 18.631
5/8" 14 0.902 1.814 22.911 20.587
3/4" 14 1.041 1.814 26.441 24.117
7/8" 14 1.189 1.814 30.201 27.877
1" 11 1.309 2.309 33.249 30.291
1-1/8" 11 1.49 2.309 37.897 34.939
1-1/4" 11 1.650 2.309 41.910 38.952
1-1/2" 11 1.882 2.309 47.803 44.845
1-3/4" 11 2.116 2.309 53.746 50.788
2" 11 2.347 2.309 59.614 56.656
2-1/4" 11 2.587 2.309 65.710 62.752
2-1/2" 11 2.960 2.309 75.189 72.226
3" 11 3.460 2.309 87.844 84.926
  • 1 inch (in) = 25.4 millimeter (mm)
  • 1 millimeter (mm) = 0.03937 inch (in)

Reference standards for BS 21 tapered pipe threads are:

  • ISO 7/1
  • DIN 2999 - Germany
  • JIS B 0203 - Japan

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