BSP Thread Couplings

BSP (British Standard Pipe) thread is the international thread standard used on pipe fittings, valve ports, pump connections and fire equipment worldwide. BSP thread is specified in ISO 228 (parallel, BSPP) and ISO 7 (taper, BSPT) and uses the Whitworth thread form with a 55° thread angle. The thread size is designated by a G (or R for taper) prefix followed by the nominal pipe size in inches — for example G2" refers to a BSP thread nominally associated with a 2-inch pipe bore, although the actual thread diameter is larger than 2".

BSP thread is used on fire hose couplings as the universal interface between the fire hose coupling system (Storz, NOR, Gost, BS336, NH etc.) and the threaded connections on pumps, hydrants, nozzles, monitors, standpipes and building fire protection systems. Without BSP thread adaptors, every coupling standard would need a dedicated threaded fitting for every pump port and appliance connection — BSP thread is the standard that makes all fire hose coupling systems interoperable with the broader plumbing and fire equipment infrastructure.

CA-FIRE manufactures BSP thread adaptors for every coupling standard in the CA-FIRE range, from G½" to G6", in brass, aluminium and stainless steel.

BSPP (British Standard Pipe Parallel) thread has a constant diameter along its length. A BSPP connection does not seal on the thread — it requires a face seal gasket or O-ring to achieve a watertight connection. The thread provides clamping force; the gasket provides the seal. BSPP is used where the connection must be assembled and disassembled repeatedly — fire hose couplings, removable pump connections and field-assembled fire hose systems. BSPP is by far the most common thread type on fire equipment and is what most people mean when they say "BSP thread".

BSPT (British Standard Pipe Taper) thread has a diameter that increases from the end toward the base. The seal is achieved by the tapered threads drawing together under torque — PTFE tape or thread sealant is applied. BSPT is used on permanently installed connections that will not be frequently disassembled — pump body ports, valve bodies and fixed pipe connections. For fire hose applications that require regular make-up and break-apart, BSPP is the correct choice.

For ordering CA-FIRE BSP thread adaptors: unless you have a specific requirement for tapered thread, specify BSPP. The vast majority of fire equipment — pump ports, hydrant valves, nozzles — use BSPP parallel thread with a face seal gasket.

BSP (British Standard Pipe) and NPT (National Pipe Taper) are different thread standards with different thread forms and dimensions — they are NOT compatible and will not seal if connected together.

The key differences: BSP uses a 55° Whitworth thread angle; NPT uses a 60° thread angle. BSP and NPT have different thread pitches for the same nominal size — a G2" BSP male will not thread correctly into a 2" NPT female socket, and attempting to force a connection may result in cross-threading or thread damage without creating a watertight seal.

CA-FIRE manufactures BSP × NPT cross-adaptors that allow BSP-threaded fire equipment to connect to NPT-threaded fittings and vice versa. These are available in G2" and G2½" in brass as standard — contact CA-FIRE for other sizes. These cross-adaptors are commonly needed on international projects where American fire equipment (NPT) must connect to European or Asian pump and valve systems (BSP), and on vessels where both standards are present.

The G prefix followed by a fraction in inches (G½", G¾", G1", G1¼", G1½", G2", G2½", G3", G4" etc.) designates the nominal pipe size of the BSP thread. This is a historical designation that refers to the approximate internal bore of the pipe that the thread is intended to seal — not the actual thread diameter. The actual thread diameter is always larger than the nominal pipe size.

For example: G2" BSP thread has an actual thread outer diameter of approximately 59.6 mm — not 2 inches (50.8 mm). The G2" designation refers to the approximate internal bore of a 2" steel pipe, not to the thread dimensions. This system of thread sizing originates from the historical practice of measuring pipe by its internal bore rather than its external diameter.

The practical implication for ordering is straightforward: match the G-size of the BSP thread adaptor to the G-size of the pump port, hydrant outlet or nozzle thread you need to connect to. If the pump outlet is marked G2½", order a G2½" BSP adaptor. If the thread size is unknown, measure the thread outer diameter and cross-reference to the BSP thread size table — CA-FIRE can advise based on measurements or photos.

The choice between male and female BSP thread adaptor depends on the thread gender of the appliance or fitting you need to connect to:

Use a male BSP adaptor (coupling + BSP external thread) when the appliance or fitting has an internal (female) thread. Common female-threaded appliances: pump discharge outlets (female G2½" or G3"), fire monitor inlets (female G2½" or G3"), nozzle inlets (female G¾" to G1½"), gate valve outlets (female), fire hose cabinet connection outlets (female).

Use a female BSP adaptor (coupling + BSP internal thread) when the appliance or fitting has an external (male) thread. Common male-threaded fittings: hydrant standpipe outlets (male), pipe nipples (male), pump inlet connections (male), some nozzle bodies (male thread for monitor connection), building standpipe male outlets.

A useful rule of thumb: fire pump discharge outlets are typically female-threaded, requiring a male adaptor on the hose end. Building standpipe and hydrant outlets vary by country and installation — when in doubt, confirm the thread gender of the appliance before ordering adaptors. CA-FIRE can advise based on the pump or equipment model number.

Yes — CA-FIRE manufactures BSP thread adaptors for all coupling standards in the CA-FIRE range:

Storz × BSP: male and female BSPP, swivel female, long male — G½" to G4½" — AL Forging, BR Forging, SS316
NOR × BSP: male and female BSPP — LAS1/LAS2/LAS3 — G1" to G3" — AL Forging, BR Forging
Gost × BSP: male and female BSPP — DN50 to DN125 — G2" to G4" — AL Forging/Casting, BR Casting
Rotta × BSP: male and female BSPP — DN50 to DN70 — G2" to G2½" — AL Casting, BR Casting
Barcelona × BSP: male and female BSPP — 1" / 1½" / 2½" — G¾" to G2½" — AL Forging/Casting, BR
BS336 BI × BSP: male and female BSPP — ¾" to 2½" — G¾" to G2½" — AL Casting, BR Casting
DSP × BSP: male and female BSPP — DSP40/DSP65/AR100 — G1½" to G4" — AL Casting
NH × BSP: male and female BSPP — 1½" to 4" — G1½" to G4" — AL Casting, BR Casting
UNI804 × BSP: male and female BSPP — Type A/B/C — G1" to G4" — AL, BR

For mixed-standard adaptors (Storz × NH, NOR × BS336, Gost × Storz etc.) or non-standard sizes, contact CA-FIRE at sales@ca-fire.com with your requirements.