BSP connections explained —
BSPP, BSPT and where the seal actually forms
A parallel flat-face joint normally seals on the face with a flat gasket. A taper thread normally seals on the thread with PTFE tape or a specified compound. If you identify the thread but miss the sealing face, you can choose the right size and still build a joint that weeps.
The two BSP types — parallel and taper
BSPP
The thread diameter stays constant along the full length. BSPP does not by itself specify the sealing element. In the flat-face union shown here, the thread brings the faces together and a separate gasket seals between them; other BSPP fittings may use an O-ring, bonded washer, cone or metal seat.
This is the pattern behind many UK heating and plumbing flat-face fittings: valves, unions, circulating pump connections, boiler fittings and appliance connections.
BSPT
The thread diameter reduces along the length, forming a 1 in 16 taper. As the male and female threads engage, the taper creates the sealing interface on the thread flanks. PTFE tape or an approved thread compound is normally used to fill the small paths between the threads.
BSPT appears in some gas, industrial and specialist connections, and on some imported equipment. Do not treat BSPT and NPT as interchangeable: the thread forms are different even when the size label looks familiar.
The common mistake: treating a BSPP flat-face joint as if the thread is the seal. Tape built up on a parallel thread can stop the two faces closing cleanly, leaving the gasket under-compressed. For a flat-face BSPP joint, the practical question is not "how much tape?" but "are the two faces clean, parallel and compressing the right gasket?"
Male and female — what each term means
Male and female describe the thread orientation, not the fitting type:
- Male BSP (external thread): the thread is on the outside of the fitting. The male end is the part that screws into the female socket. On a flat face parallel connection, the male fitting has a machined flat end face — this is the seating surface for the gasket.
- Female BSP (internal thread): the thread is on the inside of the fitting or union nut. The female socket receives the male fitting. On a flat face parallel connection, the female body has a corresponding flat seating face inside — the other side of the gasket interface.
A union connection combines both: a male fitting end, a female union nut, and a union body. The gasket sits between the male fitting face and the union body face, held under compression by the union nut.
BSPP — flat face connection
BSPT — taper thread connection
How to identify which type you have
Use a thread gauge where possible. Visual clues are useful, but a gauge is better when the connection matters. A BSP gauge confirms the pitch, and a calliper check at two points along the male thread helps separate a parallel thread from a taper thread.
Visual checks that can help — but are not definitive:
- Look along the male thread: a taper thread is noticeably narrower at the tip than at the base. A parallel thread looks the same diameter all the way along.
- Look at the male fitting end face: a BSPP flat-face connection normally has a machined flat end face: the surface that compresses the gasket. A plain taper-thread fitting often lets the thread run almost to the end.
- Check the female fitting: a BSPP flat-face socket, union or body has a flat internal seat for the gasket. A taper-thread socket is shaped to seal on the thread instead.
Many UK heating and plumbing flat-face connections are BSPP. Boiler unions, circulating pumps, radiator valves and cylinder connections commonly use a parallel thread with a separate gasket or washer. That is a strong starting assumption, not a substitute for checking the actual fitting.
BSPP vs BSPT — side by side
| Property | BSPP — Parallel | BSPT — Taper |
|---|---|---|
| Thread form | Constant diameter — parallel along length | Reducing diameter — 1:16 taper along length |
| Seal location | Flat face between fitting end and socket seat | On the thread — taper interference fit |
| Sealing method | Flat fibre gasket compressed between faces | PTFE tape or thread compound on thread |
| Also known as | G thread, BSP, BSPP | R thread, taper BSP |
| Common use | Heating and plumbing flat-face fittings — valves, unions, pump and boiler connections | Some gas, industrial and specialist threaded connections |
| On reassembly | Replace gasket | Replace PTFE tape or reapply specified thread sealant as required |
BSP sizes — nominal, actual and gasket dimensions
BSP sizing uses a nominal bore designation — the number refers to the approximate bore of the pipe, not the actual thread diameter. A 1/2 inch BSP fitting has an actual thread outer diameter of approximately 20.9 mm, not 12.7 mm. This confuses anyone approaching BSP from an imperial or metric background.
| BSP size | Thread OD (mm) | TPI | Indicative gasket OD (mm) | Indicative gasket ID (mm) | Seen in some applications |
|---|---|---|---|---|---|
| ⅜ inch BSP | 16.7 | 19 | 17.2 | 10.2 | Small valve connections, gauge ports |
| 1/2 inch BSP | 20.9 | 14 | 20.8 | 13.5 | Filling loops, pressure gauges, small valves |
| 3/4 inch BSP | 26.4 | 14 | 26.4 | 18.5 | Pump unions, boiler connections — most common |
| 1 inch BSP | 33.2 | 11 | 33.3 | 24.3 | Larger pump connections, main system valves |
| 1-1/4 inch BSP | 41.9 | 11 | 41.9 | 31.8 | Commercial heating, larger valves |
| 1-1/2 inch BSP | 47.8 | 11 | 47.8 | 36.9 | Commercial and industrial connections |
Gasket dimensions are indicative for standard flat face connections. Actual dimensions vary by manufacturer and fitting series. Always measure the fitting face before ordering gaskets for non-standard connections.
Where the gasket goes — and what happens if it is in the wrong place
On a BSPP flat-face connection, the gasket must sit squarely on the sealing face before assembly. When the union nut, socket or body is tightened, the gasket is compressed between two flat faces and forms the seal.
Common errors that prevent correct gasket seating:
- PTFE tape on a parallel thread: the thread becomes bulkier, but the sealing face is not improved. On some joints, the faces no longer close evenly and the gasket is left with poor compression.
- Gasket missing or displaced: the nut may feel tight while the faces are still metal-to-metal, or while the gasket is sitting partly off the sealing land.
- Wrong gasket size: a gasket that is too small can drop into the bore; one that is too large can fold or buckle under compression.
- Dirty or damaged face: scale, old gasket material or a radial mark on the face gives the new gasket a ready-made leak path.
Gasket selection for BSP flat face connections
Once the joint type is clear, select the gasket by the service conditions: medium, temperature, pressure, face condition and any approval requirement. The BSP size gets you into the right family. It does not choose the material by itself.
- Standard heating water: GREENSEAL PRO 180 — cellulose fibre with NBR binder, up to 140°C continuous
- Potable water (UK, WRAS required): REDSEAL PRO 110 or BLUESEAL ULTRA 350 — use within the WRAS listing scope, listed conditions and maximum water temperature
- Regulated water contact: GREENSEAL PRO 180 has supplier-sheet FDA 21 CFR 175.300 and legacy KTW references, but no WRAS listing in the mapped source. Obtain current product-specific evidence for the actual market before use.
- Higher temperature or pressure: BLUESEAL ULTRA 350 — aramid fibre, up to 250°C continuous, 100 bar
- Gas duties: require a gasket grade documented for the specific gas application and approval scope. Do not infer gas suitability from BSP size or colour alone
- Poor face condition: FLEXSEAL PRO 350 — 25% compressibility accommodates surface variation
FAQ — BSP connection basics
What is the difference between BSPP and BSPT?
BSPP is a parallel thread. On a flat-face connection, the thread pulls the faces together and the gasket seals between them. BSPT is a taper thread. The thread itself is part of the sealing interface and normally uses PTFE tape or a specified thread compound.
Where does the gasket go on a BSP fitting?
On a BSPP flat-face joint, the gasket sits between the flat end face of the male fitting and the flat seat in the socket, union or body. It does not seal on the thread. On a BSPT taper-thread joint, a flat gasket is normally not the sealing element.
How do I tell if a BSP fitting is parallel or taper?
Use a thread gauge where possible, then check the male thread with a calliper at two points. A taper thread reduces in diameter along its length. A flat-face BSPP joint also normally shows a machined face or seat for the gasket.
BSPP and BSPT can look like the same family from the outside, but they seal in different places.
On a BSPP flat-face joint, the thread clamps and the gasket seals. On a BSPT joint, the thread is part of the sealing system. Most repeat mistakes come from mixing those two ideas: adding tape where a gasket should do the work, or looking for a gasket where the thread form is doing the sealing.