BS Instantaneous Couplings

A BS Instantaneous coupling (also known as a British Instantaneous or BI coupling) is a fire hose quick-connect fitting manufactured to BS 336, the British Standard for fire hose couplings. The BI coupling uses an asymmetric bayonet connection — one half is the male plug end, the other is the female socket — and they connect by inserting the plug into the socket and rotating 90° to lock.

BS 336 couplings are the standard fire hose fitting throughout the United Kingdom, Ireland, Australia, New Zealand, South Africa, Malaysia, Singapore, India, Pakistan and many other Commonwealth and former British-influenced countries. They are also specified across much of the Middle East — Saudi Arabia, UAE, Qatar and Bahrain — where UK-trained fire services and British engineering contractors have established the BS 336 standard on fire apparatus and building fire protection systems.

BS 336 covers both BI delivery couplings (¾" to 2½") and BSRT suction couplings (2½" to 6"). The full CA-FIRE firefighting coupling range covers all major international standards alongside BS 336.

Both BI and BSRT couplings are part of the BS 336 standard but they serve different purposes and use different connection mechanisms:

BI (British Instantaneous) — asymmetric bayonet quick-connect for delivery (pressure) hose. Male plug inserts into female socket and rotates 90° to lock. Available in ¾", 1", 1½", 1¾", 2", 2½" and 3". Used on fire truck attack lines, building dry riser connections, fire hose reel systems and standpipe landing valves. The most common BS 336 coupling type.

BSRT (British Standard Round Thread) — large-bore round thread coupling for suction hose. Uses a coarse round thread (not BSP) for the mechanical strength needed on large-diameter suction hose under vacuum conditions. Available in 2½", 3", 3½", 4", 4½", 5" and 6". Used on fire pump inlet suction hose connecting to open water sources.

The two types are not interchangeable — BI and BSRT use completely different connection geometries. Always specify which type is required when ordering.

The asymmetric BI coupling design predates the Storz symmetrical design in British fire service history and was standardised by BS 336 before symmetrical couplings became widespread. The British fire service adopted and standardised the BI design, and the installed base of BS 336-compatible hydrants, dry risers, appliances and hose throughout the UK and Commonwealth creates a strong path-dependency that makes changing standards economically and operationally impractical.

The practical consequence of the asymmetric design is that hose must be made with one male and one female end rather than two identical ends. When two hose lengths with the same gender coupling need to be joined, a double-female (BIFF) or double-male (BIMM) adaptor is required. This is a well-known operational consideration in the British fire service, and double-end adaptors are carried as standard equipment on UK fire appliances.

By contrast, the Storz coupling's symmetrical design eliminates the male/female matching requirement, which is why it has become the dominant international standard for new installations. However, for projects serving the UK fire service or Commonwealth markets, BS 336 compliance remains mandatory.

Because BI couplings are asymmetric, situations arise where two hose ends with the same gender coupling need to be connected together. This requires a reversing adaptor:

Double female (BIFF): has two female socket ends. Used to connect two male-ended hose lengths together — for example, when extending a line by connecting the male end of one hose to the male end of another. Also used to connect a male pump discharge outlet to a male-ended hose coil.
Double male (BIMM): has two male plug ends. Used to connect two female-ended hose lengths together — for example, the female end of an attack hose to the female end of a hydrant-connected supply hose. Also used at hydrant standpipes where the outlet is female and the hose end is also female.

CA-FIRE manufactures BIFF and BIMM adaptors in sizes 1½", 2" and 2½" in aluminium and brass casting. Double-end adaptors are standard equipment on all UK fire appliances. See the BI Caps & Double Ends product table above for part codes.

Yes — provided both couplings conform to BS 336 and are the same nominal size and type (BI or BSRT), they will be fully interchangeable regardless of manufacturer. The BS 336 standard defines the male plug and female socket dimensions, lug positions, rotation angle and gasket seat geometry precisely enough that compliant couplings from any manufacturer will connect and seal correctly.

This interoperability is a fundamental requirement of the BS 336 standard — every UK Fire and Rescue Service must be able to connect its hose to any other brigade's hose and to any building's dry riser or standpipe outlet without adaptors. CA-FIRE BS 336 BI and BSRT couplings are dimensionally verified against BS 336 for every production batch.

Note that BSRT threads are specific to BS 336 and are not compatible with BSP, Storz thread or other standard pipe threads. Always specify "BSRT" — not "BSP" — when ordering suction coupling thread adaptors to ensure the correct thread form is supplied.

BI to BSP thread: CA-FIRE manufactures BI female socket to BSP thread (BIF) and BI male plug to BSP thread (BIM) adaptors in sizes 1" to 2½", allowing BS 336 hose to connect to BSP-threaded pump outlets, nozzles, standpipe connections and appliances. These are among the most common BS 336 accessories used in practice.

BI to Storz: BI and Storz couplings use incompatible connection geometries. A mixed BI × Storz adaptor — BI on one end, Storz on the other — is available from CA-FIRE for international projects where both standards must be accommodated. Contact our sales team for custom mixed-standard adaptors.

BI to NOR: Similarly, BI × NOR mixed adaptors are available on request for vessels and offshore platforms where BS 336 and Norwegian standards both apply. Contact CA-FIRE with the specific lug sizes required.

To receive an accurate quotation for BS 336 BI or BSRT couplings, please provide the following when contacting CA-FIRE:

1. Coupling type — BI hose tail (BIT), BI female thread (BIF), BI male thread (BIM), BI blank cap (BIC), double female (BIFF), double male (BIMM), BSRT hose tail (BSRTT), BSRT female thread (BSRTF), BSRT male thread (BSRTM) or BSRT blank cap (BSRTC).
2. Size — ¾", 1", 1½", 1¾", 2", 2½" (BI); 2½", 3", 3½", 4", 4½", 5", 6" (BSRT).
3. Gender (for BI hose tails, thread adaptors and blank caps) — female socket (F) or male plug (M).
4. Hose tail bore — internal diameter of the fire hose in mm (for hose tail couplings).
5. Thread specification — BSP thread size for BI/BSRT thread adaptors.
6. Material — aluminium casting (AL) or brass casting (BR).
7. Quantity — pieces per line item.
8. Special requirements — test certificates, project marking, custom packaging.

Contact CA-FIRE at sales@ca-fire.com, via WhatsApp, or via the contact page. We respond within 24 hours with pricing and lead time.