BS 21 - Pressure Tight Pipe Thread Joints
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Note that the BS 21 is withdrawn, superseded and replaced by BS EN 10226-1:2004, BS EN 10226-2:2005, BS EN 10226-3:2005.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
Related Topics
• Dimensions of Pipes and Tubes
Pipe, tube and fittings sizes and dimensions. Inside and outside diameters, weights and more.
• Threads
Thread data - metric, unified, ACME and others.
Related Documents
ANSI B1.20.1 - NPS - American National Standard Straight Pipe Threads
NPS pressure tight straight pipe threads with lubricant or sealer.
ANSI B1.20.1 - NPT - American National Standard Taper Pipe Threads
NPT and NPTF pipe threads - dimensions according ANSI/ASME B1.20.1/3.
EN 10226 - Pipe Threads where pressure tight joints are made on the threads - Dimensions
Pipe threads where pressure tight joints are made on the threads - Part 1: Taper external threads and parallel internal threads.
ISO 228 - Pipe Threads where Pressure Tight Joints are not made on the Threads
Requirements for thread form, dimensions, tolerances and designation for fastening pipe threads, thread sizes.
ISO 2841 - Spark Plugs Metric Threads
Metric threads spark plugs according ISO 28741.
ISO 7 - Pipe Threads where Pressure-tight Joints are made on the Threads
Pressure-tight joints made on the pipe threads.
ISO 724 - Metric Threads
Dimensions of metric threads according ISO 724.
Piping Thread Standards
Applied threaded standards used for piping connections.
Threads commonly used in Piping
Piping thread standards.
UNC and UNF - Unified Coarce and Unified Fine Inch Threads
ANSI/ASME B1.1 Unified National Thread - UNC Coarse series and UNF Fine series.
Whitworth Pipe Threads
BSPP (G) - British Standard Pipe Parallel and BSPT (R/Rp) – British Standard Tapered Pipe for pipes and tapered threads - G/R/Rp.