Outside tap thread size UK: identify BSP correctly

A new hose connector can look close to the right size and still catch after half a turn. Identify an outside tap thread size in the UK by measuring the male thread's outside diameter and checking that the connector states BSP. A reading near 26.4 mm normally means 3/4 inch BSP. That result is larger than three quarters of an inch because the label is a nominal size, not the measured diameter.

In short

Measure across the complete male thread, from one outer crest to the opposite crest. Roughly 26.44 mm is 3/4 inch BSP, 20.99 mm is 1/2 inch BSP and 33.25 mm is 1 inch BSP. Match the nominal size and the thread standard, fit the correct washer, and stop if a connector binds rather than forcing it.

What BSP means on an outside tap

BSP stands for British Standard Pipe, a family of pipe threads widely used for UK plumbing connections. On the familiar hose-union outside tap, the visible male thread is commonly 3/4 inch BSP. That nominal size is a name from pipe-sizing practice, not a promise that the outside of the thread measures exactly three quarters of an inch.

Three quarters of an inch is 19.05 mm, but a male 3/4 inch BSP thread is about 26.44 mm across. A 1/2 inch BSP thread is about 20.99 mm externally, not 12.7 mm. About 26.44 mm is slightly over one literal inch, and the nominal size is still 3/4 inch BSP. A 1 inch BSP male thread is about 33.25 mm. Write down the millimetres before you guess an imperial label.

A label that only says 3/4 inch is ambiguous, because BSP also sets the thread form and pitch. Look for 3/4 inch BSP, G3/4, or a stated UK outside-tap fit, and check the manufacturer's dimensions where they are given. Universal describes a seller's range. It does not prove the fitting suits every tap.

How to measure an outside tap thread

Turn the tap off, remove the connector, and wipe soil, limescale and old washer fragments off the thread. Measure the male thread at its widest point, crest to crest, straight across the centre. Callipers are clearest. A rigid metric ruler can still separate roughly 21 mm from roughly 26.5 mm.

Do not measure the smooth section behind the thread, the bore, or one side from the bore to the edge. Hold the tool square. An angled ruler can add enough width to confuse a borderline reading. Take two measurements at right angles if the thread is worn or painted.

• A reading around 20.99 mm indicates a 1/2 inch BSP male thread.
• A reading around 26.44 mm indicates a 3/4 inch BSP male thread.
• A reading around 33.25 mm indicates a 1 inch BSP male thread.

Treat these as identification dimensions, not a machining check. Paint and corrosion can shift a reading by a fraction of a millimetre. They should not turn 21 mm into 26 mm. If the figure is nowhere near these values, measure again before you buy.

Check the thread condition as well as its size

Look for flattened crests, a split at the outlet or heavy corrosion. A correctly sized plastic connector should engage by hand for several turns without rocking or suddenly tightening. Damage can make the right standard feel wrong, and soft plastic can hide the problem until it leaks.

Counting threads as a supporting check

Diameter is the easier household check. Pitch supports it: 3/4 inch BSP has 14 threads per inch. A short, worn thread is hard to count with a ruler, and diameter alone cannot separate every similar standard. Read the product specification as well.

What to do with an older 1/2 inch tap

Some older UK outside taps have a 1/2 inch BSP outlet of about 21 mm across the thread. A dual-size connector often fits 3/4 inch BSP in the body, with a removable insert for 1/2 inch BSP. Check which way the adaptor is fitted. Photographs do not always show whether the insert arrives installed.

The washer still has to lie flat. Two loose washers, or an adaptor sitting crookedly, distort the seal if you tighten harder. Assemble the parts on a bench, then screw the connector on by hand.

A bib tap is not automatically a hose-union tap. Bib means a wall-mounted tap with the outlet pointing down. Some have a plain nose, or a spout that has to come off before a thread is available. If that thread is not already clear, ask a plumber rather than gripping the tap body with large tools.

Why GHT does not fit BSP

GHT means Garden Hose Thread, used particularly on North American hose equipment. The 3/4 inch label looks like 3/4 inch BSP, and the diameters are close enough that a female fitting may start. The pitch differs: 3/4 inch GHT is commonly 11.5 threads per inch, while 3/4 inch BSP uses 14.

Those pitches cannot stay aligned as the parts turn. A mismatched fitting may engage, lean, then bind. Forcing it can cross-thread plastic or damage the tap's metal thread. Even if it seems tight, the washer may not meet the tap square.

A listing that mentions US taps, GHT or 11.5 TPI should not be assumed to fit a UK BSP outlet. One that states 3/4 inch BSP is clearer. If you already own GHT equipment, use an adaptor sold for that direction, such as female BSP to male GHT. The spray-hose buying guide covers the other fitting checks before you choose a set.

Choosing a tap connector and spray gun

The part on the outside tap is usually a tap connector, not the spray gun. The threaded female end matches the tap. The other end is a quick-connect for the hose. A water-stop connector at the gun end stops the flow when you take the accessory off.

Record what you measured: 3/4 inch BSP at about 26.4 mm, 1/2 inch BSP at about 21 mm, or another stated standard. The tap connector should name that standard. The hose couplings and the gun then need the same quick-connect. A correct tap thread does not make every downstream coupling latch.

Confirm whether a stated thread is on the tap connector or the gun. In the spray-hose comparison, the Onarway metal spray gun is the top pick, about £15.74, with 4.7 from 262 amazon.co.uk reviews. It states ten patterns, including mist, shower, flat and jet, and a 1/2 inch quick-connect. It does not state BSP or GHT. The spray-hose use guide covers a set you have already matched. A qualifying purchase can earn a commission at no extra cost to you. Figures are from that listing, not a lab test.

Fixing leaks without damaging the thread

A leak at the tap does not mean the thread size is wrong. In a typical removable female connector, a flat washer seals against the end of the tap. The threads pull the faces together. A missing, hardened, split or crooked washer leaks even when both parts are 3/4 inch BSP.

Turn the tap off, inspect the washer, and replace a cracked or flattened one. Clean grit from the seat and refit by hand. Open the tap gradually. Hand-tight, following the instructions, is safer than pliers, which can crack a plastic collar.

The collar does not have to touch the tap body if the washer is square and the fitting is secure. If it stops after a partial turn or sits tilted, remove it and recheck the standard.

PTFE tape is not the default on a washer-sealed hose connector. It suits joints where the thread itself helps the seal. It will not fix a bad pitch or a damaged washer, and it can hide poor engagement.

If water comes from the tap body, the wall joint or the spindle, the hose fitting may not be the cause. Stop if the tap moves in the wall, the pipework strains, or frost damage is visible. Swapping a removable connector is a household job. Fixed pipework, and a bib tap you cannot identify, is a plumber's job.

Take removable connectors off before freezing weather, shut off and drain the outdoor supply where you can, and store the connector dry. The right BSP size does not stop water-filled plastic splitting in a freeze.