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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.

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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

Note that the BS 21 is withdrawn, superseded and replaced by BS EN 10226-1:2004, BS EN 10226-2:2005, BS EN 10226-3:2005.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 1^o47'
truncation of roots and crests are rounded
55^o 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) Load Calculator!	Major Diameter (mm) Load Calculator!	Minor Diameter (mm) Load Calculator!
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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Related Topics

Dimensions - Sizes and dimensions of pipes and tubes, and their fittings - inside and outside diameter, weight 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.
Standards Applied for Piping Threads - Applied threaded standards used for piping connections.
Threads commonly used in Piping - Piping thread standards.
UNC and UNF - Unified Inch Screw 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.

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