How to ID British (BSP) Threads

British Pipe Standard fittings are amongst 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°. NOTE: It is a common mistake to identify a BSPT (tapered) as an NPT. Remember that NPT threads have a 60° thread flank angle, and BSPT has a 55° angle (this can be verified with a thread gauge). Although BSP is a foreign thread, it isn't metric. This is why it comes in imperial sizes: 1/8, 1/4, 1/2, 3/4, and so on.

For tapered BSP threads, a seal is acquired via thread wedging with additional support from thread dope or Teflon® tape. 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).

You must find the I.D. to determine the threads per inch count for BSPT fittings. Simply obtain a caliper reading from inside the bore of the fitting. You can cross-reference the bore size on the following chart with thread pitch and threads per inch to find your desired size and its related specs.

Thread conforms to ISO 7

Pitch and diameter are measured in inches, e.g. G3/8-19

Tapered thread profile seals by metal to metal intererence fit, usually requires sealing compound for pressure tight connection

Taper angle is 1° 47', the same as NPT(F)

Thread angle is 55°

Not interchangeable with NPT(F)

BSPT Thread Specification (BSPT Indicator: R)
Nominal Size (in)	Dash Size	Outer Diameter (mm)	Inner Diameter Male Thread (mm)	Pitch (mm)	Threads per Inch	Pitch Diameter	Thread Height (mm)	Tap Drill Diameter (mm)	Thread Crest Rounding
1/16	01	7.723	6.561	0.907	28	7.142	0.581	6.40	0.125
1/8	02	9.728	8.566	0.907	28	9.147	0.581	8.40	0.125
1/4	04	13.157	11.445	1.337	19	12.301	0.856	11.20	0.184
3/8	06	16.662	14.950	1.337	19	15.806	0.856	14.75	1.840
1/2	08	20.955	18.631	1.814	14	19.793	1.162	18.25	0.249
3/4	12	26.441	24.117	1.814	14	25.279	1.162	23.75	0.249
1	16	33.249	30.291	2.309	11	31.770	1.479	30.00	0.317
1¼	20	41.910	38.952	2.309	11	40.431	1.479	38.5	0.317
1½	24	47.803	44.845	2.309	11	46.324	1.479	44.5	0.317
2	32	59.614	56.656	2.309	11	58.135	1.479	56.0	0.317

Thread Size = Male Thread OD - Threads per Inch

BSPT	T.F.F.T.
1/8-28	2 - 3
1/4-19	2 - 3
3/8-19	2 - 3
1/2-14	2 - 3
3/4-14	2 - 3
1-11	1.5 - 2.5
1¼-11	1.5 - 2.5
1½-11	1.5 - 2.5
2-1	1.5 - 2.5

Assembly Turns From Finger Tight values
for steel, stainless steel and brass fittings.