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British Standard Pipe Parallel Thread conforms to ISO 228-1 Port conforms to ISO 1179 Pitch & diameter measured in inches e.g. G1/4-19 Parallel threads require o-ring, crush washer, gasket or metal to metal seal between connectinos for pressure tight connection Thread angle is 55° Not interchangeable with SAE or NPT(F)

BSPP Thread Specification (BSPP Indicator: G) Size Nominal Size (in) Thread Size (mm) 2 1/8 9.728 - 28 4 1/4 13.157 - 19 6 3/8 16.662 - 19 8 1/2 20.955 - 14 10 5/8 22.911 - 14 12 3/4 26.441 - 14 14 7/8 30.201 - 14 16 1 33.249 - 11 18 1-1/8 37.897 - 11 20 1-1/4 41.910 - 11 22 1-3/8 44.323 - 11 24 1-1/2 47.803 - 11 28 1-3/4 53.746 - 11 32 2 59.614 - 11 Thread Size = Male Thread OD - Threads per Inch

How to ID British Threads
British Pipe Standard fittings are among the most popular of all foreign threads in the world. These threads come in 2 versions: parallel (BSPP), and tapered (BSPT). The thread flank angle for both tapered and parallel British threads is 55°. Although BSP is a foreign thread, it isn't actually metric. This is why it comes in imperial sizes: 1/8, 1/4, 1/2, 3/4, and so on.
BSP parallel threads commonly seal via a 30° chamfer on the male thread to a 30° recessed cone inside the female thread (swivel only). If it is a port application, an o-ring & washer or a bonded washer are needed to achieve a proper seal for parallel threads. It is possible for a male BSPT (tapered) to thread into a female BSPP (parallel), so long as the female thread is fixed and not swivel (this is because of the recessed cone seat on the BSPP end).
BSP parallel fittings and their specifications can be identified by doing a few calculations. To find the thread size:

1. Measure the O.D. (outer diameter) of the BSP thread.
2. Take the O.D. measurement (in inches) and subtract ¼".

For example, a BSP parallel male thread measures out to O.D. 1". Subtracting ¼ gives a thread size of ¾, also known as dash 12.
Once you have the thread size, you then need to determine the number of threads per inch to verify that it is a BSP fitting. You can do this by counting the number of thread crests over a ¼" length, then multiply it by 4 to get the number of threads per inch. For example, if over a ¼" distance you find 4.5 thread crests:

1. Multiply 4.5 by 4 to get 18. This is the number of threads/inch.
2. Combine the thread size with the number of threads per inch and refer to the following chart to find your desired size and its related specifications.

Nominal Size (in)	Dash Size	Outer Diameter (mm)	Inner Diameter Male Thread (mm)	Pitch (mm)	Threads per Inch	Pitch Diameter (mm)	Thread Height (mm)	Tap Drill Diameter (mm)
1/8	02	9.728	8.566	0.907	28	9.147	0.581	8.70
1/4	04	13.157	11.445	1.337	19	12.301	0.856	11.60
3/8	06	16.662	14.950	1.337	19	15.806	0.856	15.00
1/2	08	20.955	18.631	1.814	14	19.793	1.162	19.00
5/8	10	22.911	20.587	1.814	14	21.749	1.162	20.75
3/4	12	26.441	24.117	1.814	14	25.279	1.162	24.50
7/8	14	30.201	27.877	1.814	14	29.039	1.162	28.00
1	16	33.249	30.291	2.309	11	31.770	1.479	30.50
1-1/8	18	37.897	34.939	2.309	11	36.418	1.479	35.00
1-1/4	20	41.910	38.952	2.309	11	40.431	1.479	39.50
1-3/8	22	44.323	41.365	2.309	11	42.844	1.479	41.50
1-1/2	24	47.803	44.845	2.309	11	46.324	1.479	45.00
1-3/4	28	53.746	50.788	2.309	11	52.267	1.479	51.00
2	32	59.614	56.656	2.309	11	58.135	1.479	57.00

Dash Size	BSPP Thread G Size	Assembly Torque Nm +10% -0
		Straight Male Stud Fittings			Non-Return Valves RHV/RHZ	Plugs VSTI- Integrated	Straight and Adjustable Fittings
		Sealing Washer1	Cutting Face2	Integrated Sealing3	Integrated Sealing3	Integrated Sealing3	O-ring with Retaining Ring4 and Bonded Washer1
02	G 1/8A	9	18	18	18	13	18
04	G 1/4A	35	35	35	35	30	35
06	G 3/8A	45	70	70	50	60	70
08	G 1/2A	65	140	90	85	80	90
12	G 3/4A	90	180	180	140	140	180
16	G 1A	150	330	310	190	200	310
20	G 1¼A	240	540	450	360	450	450
24	G 1½A	290	630	540	540	450	540