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Note that the BS 21 is withdrawn, superseded and replaced by BS EN 10226-1:2004, BS EN 10226-2:2005, BS EN 10226-3:2005.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
| 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
