BSP Brass Fittings: Thread Sizes, Fitting Types, and Trade Applications

BSP brass fittings are the connective tissue of UK plumbing. Every boiler, every valve, every appliance connection uses them. But get the wrong thread standard, the wrong size, or the wrong fitting type and you’re either leaking or calling back the next day. This guide covers what you need to know to spec and fit BSP brass correctly.

BSP: What the Standard Means

BSP stands for British Standard Pipe. It’s the thread standard used on virtually all UK plumbing fittings, boilers, valves, radiators, and appliances. There are two variants:

BSPP (Parallel) — the thread form is cylindrical. The thread diameter is the same from start to finish. The seal is made by a flat washer, O-ring, or sealant at the mating face, not by the thread itself. Also called “G” thread (ISO 228). This is what you’ll use on 99% of plumbing work — boiler connections, radiator valves, tank fittings, compression adapters.

BSPT (Taper) — the thread tapers (narrows) along its length. The seal is made by the thread itself as it’s tightened, wedging into the mating thread. Also called “R” thread (ISO 7). Used in gas pipework, some industrial fittings, and where the joint must self-seal without a separate sealer. Less common in domestic plumbing.

In practice, most plumbing fittings marked “BSP” in trade supply are parallel (BSPP). Always use PTFE tape, hemp, or liquid thread sealant on parallel threads — the thread alone won’t seal without it.

BSP Size Chart: What the Numbers Mean

BSP sizes are nominal — they don’t directly correspond to the actual thread diameter. This catches out anyone new to the trade. The nominal size approximates the bore of the pipe the thread was originally designed for, not the OD of the thread.

Common sizes and their approximate thread ODs:

• 1/8" BSP — thread OD approx 9.7mm. Used for pressure gauge connections, drain plugs, small instrument fittings.
• 1/4" BSP — thread OD approx 13.2mm. Pressure tappings, some shower fittings, condensate connections.
• 3/8" BSP — thread OD approx 16.7mm. Basin tap tails (older pattern), some inline fittings.
• 1/2" BSP — thread OD approx 20.9mm. The most common domestic size. Tap tails, radiator valves, isolation valves, boiler connections, flexible hoses.
• 3/4" BSP — thread OD approx 26.4mm. Cold mains connections, gate valves, filling loops, larger appliance inlets.
• 1" BSP — thread OD approx 33.2mm. Cylinder connections, boiler primary connections on larger plant, some pump bodies.
• 1¼" and 1½" BSP — larger cylinder, tank, and commercial fittings.
• 2" BSP — tank connections, commercial plant.

When in doubt, use thread gauges or compare against a known fitting. Never guess on gas connections.

Male vs Female: Telling Them Apart

Male BSP (MI — Male Iron, or M) has external threads. Female BSP (FI — Female Iron, or F) has internal threads. On a fitting description, “M x F” means one male end and one female end. “F x F” means both ends female.

A common source of confusion: tap tail connections. Most modern tap tails are male 1/2" BSP — they screw into a female fitting (backnut or flexi hose). The flexible tap connector screws onto the tap tail from below. Get this backwards and nothing will thread together.

Key Fitting Types

Hexagon nipple (M x M) — external threads on both ends, hexagon body for spanner grip. Joins two female-threaded components. The most common BSP fitting in the trade. Available in equal sizes (e.g. 1/2" x 1/2") and reducing versions (e.g. 3/4" x 1/2"). APM stocks Embrass Peerless hex nipples in 1/2", 3/4", and 1".

Running nipple — similar to a hex nipple but longer bodied, allowing adjustment in tight spaces. The Primaflow 1/2" Brass Running Nipple is the go-to for making up close connections where a standard nipple is too short.

Socket (F x F) — internal threads on both ends, hex body. Joins two male-threaded components. Primaflow stocks sockets in 1/2" chrome and smaller sizes (1/8", 1/4", 3/8") for instrument and gauge connections.

Tee (F x F x F) — three-way branch fitting. Used where a circuit splits into two directions or where a gauge tapping is needed off a main run. Embrass Peerless brass tees in 1/2" and 3/4".

Hexagon bush — male thread on the outside, female thread on the inside at a smaller size. Used to reduce from a larger to a smaller thread within a single fitting — e.g. a 3/4" female port reduced to accept a 1/2" male fitting. Embrass Peerless stocks bushes in 3/4" x 1/2", 1" x 3/4", and 1" x 1/2".

Cap — female thread, blanks off a male BSP end permanently or temporarily. The Embrass Peerless 1/2" Brass Cap is used to decommission a tapping, cap a drain point, or blank a gauge port during testing. Also available in 3/4" and 1".

Flanged plug — male thread, blanks off a female port. The flanged head seats against the fitting face for a flush, neat finish. Used on radiator blanking plugs, cylinder bosses, and drain tappings.

Backnut — threaded nut with a flat bearing face. Secures a fitting through a panel or tank wall by threading onto the male tail from the back, clamping both sides of the panel. The Embrass Peerless 1/2" Heavy Pattern Backnuts are the standard for tap tail connections — the heavy pattern has a wider flange and is less likely to crack thin panels than standard weight.

Tap tail extension (M x F) — extends a short tap tail to reach the underside connection. Essential on thick stone, solid wood, or composite worktops where the standard 10–12mm tail length won’t reach the flexi hose below. Embrass Peerless tap tail extensions in 1/2" and 3/4".

Sealing BSP Threads

Parallel BSP threads (BSPP) do not self-seal. You must apply thread sealant:

• PTFE tape — wrap clockwise (looking down the male thread) 3–5 turns for water, 10+ turns for gas. Standard for most connections. Does not harden, so the joint can be remade.
• Hemp (flax) and paste — traditional method still widely used by gas engineers and plumbers. Hemp is wound onto the thread and impregnated with jointing compound. More bulk than PTFE, good for slightly worn or oversized threads.
• Liquid thread sealant (Boss White, Loctite 55, etc.) — applied to the thread and cures to a semi-flexible seal. Works well on difficult threads, PTFE-incompatible materials, and gas where regulations require a non-stripping sealant.

Never use PTFE tape or hemp on face-seal connections (joints that seal on a flat washer or O-ring). The sealant goes on the thread; the face must be clean and undamaged for the washer to seat.

Brass BSP vs Compression vs Push-Fit

BSP threaded fittings are for connecting to appliances, valves, and equipment with threaded ports. You cannot compression-fit onto a threaded port — you need a BSP male or female end to mate with it.

Compression fittings grip the outside of copper tube. Push-fit fittings grip plastic or copper tube. Neither of these connects to a threaded port without an adapter. BSP-to-compression and BSP-to-push-fit adapters bridge the systems where a copper or plastic pipe run needs to terminate at a threaded valve or appliance.

Stock Available in Acton Today

APM Electricals stocks the full Embrass Peerless and Primaflow BSP brass fittings range — nipples, sockets, tees, bushes, caps, plugs, backnuts, and tap extensions — in 1/8" through 2" BSP, from the trade counter at 24 Western Avenue, Acton W3 7TZ. Call 020 8702 8080 to check stock. Same-day collection, Monday to Friday.