Pressure Tight Thread Joints BS 21
BSPT - British Standard Pipe Taper threads & BSP or BSPF - British Standard Pipe Parallel (Straight) threads
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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
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
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