3/8" BSPT Thread
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3/8" BSPT (R 3/8") thread British tapered thread in power hydraulics and pneumatics
3/8" BSPT thread (short for British Standard Pipe Tapered, in standardized markings appearing as R 3/8" for the external male thread and Rc 3/8" for the internal female thread) is a Whitworth profile British tapered pipe thread commonly used in Europe and Asia, manufactured according to ISO 7-1, EN 10226, DIN 2999, and BS 21\. It plays a key role in power hydraulics, industrial pneumatics, measuring apparatus, and gas-liquid installations. It is characterized by a tapered profile with a 1:16 (6.25%) taper and a thread profile angle of 55°. The seal in this system is purely mechanical on the threads – during tightening, the male tapered thread deforms and wedges into the Rc 3/8" tapered socket or Rp 3/8" (G 3/8") cylindrical socket, creating a tight thread connection. For the 3/8" BSPT size, the thread pitch is 19 threads per inch (19 TPI / pitch ~1.337 mm). The outer diameter of the male thread measured with a caliper at the beginning of the taper ranges from 16.3 mm to 16.6 mm, and the internal diameter of the female thread is from 14.9 mm to 15.4 mm. Connectors and fittings with 3/8" BSPT thread are a constant standard in gear pump bodies, manifold blocks, cylinders, pressure gauges, and in pressure lines of agricultural and construction machinery.
Construction, sealing mechanism, and application in pressure systems
The primary task of the 3/8" BSPT thread is to ensure complete leak-free operation of the main line without the need for face seals, soft sealing washers, or O-rings. Due to the tapered geometry, this connection handles pressure pulsations and mechanical vibrations occurring during machine operation in harsh conditions very well.
- Self-sealing 1:16 taper: Tightening the R 3/8" male thread causes the thread surfaces to match over the entire working length, which guarantees high pressure resistance.
- 55° Whitworth profile: Traditional European pipe thread profile angle, constituting the basic feature distinguishing it from the American 3/8" NPT tapered thread (where the profile angle is 60° and the pitch is 18 TPI).
- Caliper verification dimensions: Male (external) thread measures 16.3–16.6 mm at the inlet, while the female (internal) thread shows a hole diameter of 14.9–15.4 mm with a pitch of 19 threads per inch.
- Wide range of applications: 3/8" BSPT components are used in pressure gauge connections, shut-off valves, pump valve blocks, pneumatic guns, and connections for crimping pressure hoses (AGR-K / C31).
- Sealing support method: In workshop practice, when assembling BSPT threads, it is recommended to use PTFE Teflon tape, hemp with sealing paste, or dedicated anaerobic adhesives for power hydraulics.
What to combine 3/8" BSPT thread with and how to avoid assembly errors?
The 3/8" BSPT external tapered thread (marked in crimping catalogs as AGR-K or C31) directly cooperates with Rc 3/8" internal tapered sockets and G 3/8" (Rp 3/8") perpendicular cylindrical threads. When building and regenerating flexible installations, AGR-K ends are crimped onto one- and two-braid hoses (1SN, 2SN, 1SC, 2SC) using universal ferrule sleeves. In rigid installations, the R 3/8" thread connects with straight, elbow, and tee adapters transitioning to a metric system with a 24° cone (e.g., CEL / DKOL) or American JIC / ORFS connections.
The most common error when selecting components is confusing the 3/8" BSPT tapered thread with the American 3/8" NPT thread. Although the outer diameters of both threads are similar (~16.5 mm), they differ in pitch (19 TPI in BSPT vs 18 TPI in NPT) and profile angle (55° vs 60°). Attempting to screw an NPT thread into a BSPT socket leads to thread galling and permanent leakage. Equally important is distinguishing the tapered BSPT thread from the cylindrical BSPP (G 3/8") thread – a cylindrical thread does not seal on the threads and requires an ED face seal or a bonded seal (metal-rubber washer) for proper operation.
Technical specification
Technical parameter | Value / Description |
|---|---|
Thread designation | 3/8" BSPT / R 3/8" (male tapered thread) / Rc 3/8" (female tapered thread) |
International manufacturing standards | ISO 7-1, EN 10226, DIN 2999, BS 21 |
Outer diameter of male thread (caliper) | 16.3 mm – 16.6 mm |
Inner diameter of female thread (caliper) | 14.9 mm – 15.4 mm |
Thread pitch (number of threads per inch) | 19 threads per inch (19 TPI / pitch \~1.337 mm) |
Thread profile angle and taper | 55° (Whitworth profile), 1:16 taper (6.25% convergence) |
Crimping codes for hoses and adapters | AGR-K (C31 - male tapered), BB/AB/AA BSPT fittings, screw-in connectors |
Available material versions | Galvanized free-cutting steel, AISI 316L stainless steel, brass |
Frequently Asked Questions (FAQ)
How to correctly measure a 3/8" BSPT thread with a caliper?
Placing the caliper on the external male thread at the beginning of the taper gives a result within 16.3–16.6 mm. When measuring the internal (female) thread, the hole diameter is between 14.9 mm and 15.4 mm, and a thread gauge will show 19 threads per inch.
What is the difference between a 3/8" BSPT thread and a 3/8" BSPP (G 3/8") thread?
The BSPT (R 3/8") thread has a 1:16 tapered profile and seals directly on the thread turns. The BSPP (G 3/8") thread is a cylindrical (straight) thread and seals on the front face using a bonded seal or an ED screw-in seal.
Can a 3/8" BSPT thread be screwed into a 3/8" NPT socket?
No. The BSPT thread has a profile angle of 55° and a pitch of 19 threads per inch, while the American NPT thread has a profile angle of 60° and a pitch of 18 threads per inch. Despite similar diameters, the differences in pitch and profile angle cause thread damage and leaks.
How is a connection with a 3/8" BSPT tapered thread sealed?
BSPT tapered connections are sealed on the threads by wrapping PTFE Teflon tape, using hemp with sealing paste, or using a liquid anaerobic sealant resistant to high pressures and hydraulic oils.