What is a BSPT (R) thread?

BSPT (British Standard Pipe Taper, designation R) is a tapered pipe thread with a 1:16 taper and a 55° Whitworth profile defined by ISO 7-1. The threads seal by interference as they tighten, usually with the help of a thread sealant.

Is BSPT compatible with NPT?

No. BSPT uses a 55° thread angle and NPT uses 60°, and their thread pitches differ, so they do not seal reliably together and can damage each other. Confirm the standard before assembling tapered pipe fittings.

When should I use BSPT instead of BSPP?

Use BSPT (tapered, R) where the connection must seal on the thread itself, typically in pipework following British/ISO standards. Use BSPP (parallel, G) where sealing is provided by a washer, O-ring or face seat and the thread only provides mechanical retention.

Thread Tables

ISO 7-1

BSPT Thread — British Standard Pipe Taper

Tapered, seals by thread — 55° Whitworth angle, NOT interchangeable with NPT

BSPT (British Standard Pipe Taper), also called R or PT, is the ISO 7-1 standard for tapered pipe connections with 55° Whitworth flank angle. It seals by thread interference, similar to NPT — but the two systems are INCOMPATIBLE due to different flank angles (55° vs 60°).

Dimension table

Designation (R)	TPI	Ref. OD (mm)	Pitch (mm)	d₂ at gauge plane (mm)	Taper
R 1/8	28	9.728	0.907	9.147	1:16
R 1/4	19	13.157	1.337	12.301	1:16
R 3/8	19	16.662	1.337	15.806	1:16
R 1/2	14	20.955	1.814	19.794	1:16
R 3/4	14	26.441	1.814	25.280	1:16
R 1	11	33.249	2.309	31.770	1:16
R 1-1/4	11	41.910	2.309	40.431	1:16
R 1-1/2	11	47.803	2.309	46.324	1:16
R 2	11	59.614	2.309	58.135	1:16
R 2-1/2	11	75.184	2.309	73.705	1:16
R 3	11	87.884	2.309	86.405	1:16
R 4	11	113.030	2.309	111.551	1:16
R 5	11	138.430	2.309	136.951	1:16
R 6	11	163.830	2.309	162.351	1:16

Source: ISO 7-1:1994. Ref. OD = outer diameter at gauge plane. d₂ calculated with Whitworth 55° formula: d − 0.6403×P. The 1:16 taper ratio is the same as NPT, but the 55° angle makes the systems incompatible.

Key technical characteristics

Flank angle: 55° (Whitworth)
Profile: Tapered
Standard: ISO 7-1 (also BS 21)
Designation: R (male) / Rc (tapered female) / Rp (parallel female)
Sealing mechanism: Thread interference + sealant
Taper: 1:16 (same ratio as NPT)

Typical applications

Hydraulic and pneumatic equipment manufactured in Europe (Germany, Italy, UK).
Parker, Festo, Rexroth and other European pneumatic manufacturers.
Lubrication connections in Mori Seiki, Okuma lathes and milling machines (some models).
Gas and steam systems in industrial plants under BSP standard.

Compatibility and interoperability

✗CRITICAL: NOT interchangeable with NPT even though both taper at 1:16. BSPT 55° Whitworth vs NPT 60° makes profiles incompatible. Forced assembly may appear sealed but will leak under pressure.
✗NOT interchangeable with BSPP (G) — same BSP family but BSPT is tapered and BSPP is parallel.
✓BSPT male (R) enters BSPP parallel female (Rp) — common in European fittings. Sealing depends on radial compression. Verify pressure rating with manufacturer.

Selection guide

01Before buying a pipe fitting for European machines: measure thread angle with a thread gauge — 60° = NPT, 55° = BSP.
02PT designation (used in Japan and Korea) is equivalent to BSPT — same ISO 7-1 geometry.
03Use PTFE tape or thread compound on BSPT as with NPT — interference alone does not guarantee sealing at high pressures.
04For new designs in Mexico prefer NPT (more locally available) or BSPP with face o-ring (easier to seal).

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