BSP, NPT & ACME Thread Buyer's Guide
UK Flow Centre buying guide
Choose the right connection before you order
BSP, NPT and ACME are different thread systems. Check the size, thread type and seal at both ends, especially when buying a swing joint or replacing a sprinkler.
A fitting that starts to screw in is not proof of a correct match. Stop if it binds, sits incorrectly or needs force.
Start here: what am I looking at?
Male thread: the thread is on the outside of the fitting. Female thread: the thread is inside the opening. These words describe the connection, not its size or thread standard.
This Hansen nipple has external threads at both ends. BSPT male threads taper towards the tip. The taper is slight and may be difficult to judge from a photograph.
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The threads are inside this Hansen socket. A female connection still needs the correct size, thread type and seal.
View this productThese are real catalogue photographs. Tap a photograph to enlarge it. Photographs help with recognition; measurements, markings and the exact product specification confirm the match.
Holding an unidentified fitting? Start with measuring the thread. If the thread is inside a sprinkler body, use the model and inlet specification or ask us to identify it.
1. What are BSP, BSPP, BSPT and NPT?
BSP means British Standard Pipe. The common descriptions are BSPP for parallel and BSPT for tapered. NPT is a separate American pipe-thread standard. It is not a type of BSP.
| Marking | What it means | What to check |
|---|---|---|
| BSPP / G | Parallel thread to ISO 228-1. Its diameter stays constant along the thread. | The connection normally seals at a washer, O-ring or designed seat, rather than on the thread itself. |
| BSPT / R | Tapered male pipe thread to ISO 7-1. Its diameter changes along the thread. | Use the matching female connection and the manufacturer's specified thread sealant. |
| Rp / Rc | ISO 7-1 female connections: Rp is parallel; Rc is tapered. | These are intended to mate with an R male thread for a joint that seals at the threads. |
| NPT | American tapered pipe thread, commonly specified to ASME B1.20.1. | Match it to NPT. Similar-looking BSP threads are not a substitute. |
Parallel does not always mean G/BSPP: an Rp female thread is also parallel, but belongs to the ISO 7-1 sealing system. Do not choose the seal from the word “parallel” alone. If a listing only says BSP, confirm the detailed thread form when it matters to the connection.
Technical references: Parker fitting-thread guide (PDF) and BAER pipe-thread designations.
See Parker’s manufacturer illustrations of parallel and tapered threads and the calliper check (PDF).
Why BSP and NPT should not be mixed
BSP uses a 55° thread form; NPT uses 60°. Pitch and dimensions also differ, although some sizes look very similar. For example, 1-inch BSP has 11 threads per inch and 1-inch NPT has 11½. Use a correctly specified adaptor when changing standards.
2. ACME sprinkler threads and internal swing-joint threads
ACME is a different thread form used on many larger irrigation sprinklers and swing-joint outlets. In these connections, an O-ring provides the water seal. Match the nominal size and the exact compatible inlet and outlet.
This is the ACME end of our 1-inch BSP to 1¼-inch ACME adaptor. The black ring near the end seals against the matching sprinkler inlet; the thread holds the connection together.
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Turn the adaptor over and the female BSP thread is inside the opening. This example shows why the two ends of one fitting need to be identified separately.
View this productThere are two different connections to recognise:
The outlet that joins the sprinkler
This is the end designed to connect to a compatible ACME sprinkler inlet. Many familiar irrigation ACME outlets use one sealing O-ring.
Check the outlet size and manufacturer's compatibility information. The word ACME alone does not guarantee a match across every component or brand.
The swivel inside the swing joint
These connections join the swing-joint components to each other. At UK Flow Centre, we refer to these as internal ACME connections.
This is workshop terminology for their position in the assembly. It does not simply mean a female thread, and it is not a universal compatibility standard.
Two O-rings? Check before buying
On the Rain Bird swing-joint components we are describing, two O-rings are a useful clue that you are looking at the internal swivel connection. Do not use that internal swivel end as a direct substitute for a sprinkler ACME outlet. It needs its matching swing-joint component.
O-ring count is a warning sign, not a universal identification test. One O-ring does not prove compatibility either. Check the complete part number, thread geometry and sealing arrangement.
Rain Bird specifies double O-ring protection for its swing joints and a modified ACME sprinkler outlet. Its parts catalogue separately lists an ACME outlet seal and internal front and back O-rings. A complete swing joint with the correct outlet can connect to the intended sprinkler; the internal swivel parts are a different connection.
Manufacturer references: Rain Bird swing joints, ACME outlet seal, internal front O-ring and internal back O-ring.
3. Buying a swing joint or replacement sprinkler
- Identify the sprinkler. Use the exact model, body marking and selected variant. A brand name or installation year is not enough.
- Check its inlet. Record the nominal size, thread standard and whether the connection is male or female.
- Check the supply end. The swing-joint inlet must match the service tee or pipe fitting.
- Check the sprinkler end. The swing-joint outlet must match the sprinkler inlet. The two ends of a swing joint can have different sizes and thread types.
- Check the assembly. Confirm lay-arm length, bore, outlet arrangement and any extra elbow. An internal replacement elbow must match that swing-joint range.
In this photograph, the upper-left end connects to the supply. The lower-right end, with the O-ring near its tip, connects to the sprinkler. The swivel joints between them have their own connections.
View this productFor example, a 1-inch BSP inlet × 1¼-inch ACME outlet swing joint has two different connections. The 1¼-inch ACME end is the sprinkler end. Neither its lay length nor its nominal arm diameter identifies both threads.
Some sprinkler listings offer BSP and ACME variants on the same page. Choose the inlet you actually need. For an existing BSP swing joint and an ACME sprinkler, select an approved adaptor with the correct sizes and connection direction.
Shop swing joints · Shop sprinklers · Shop threaded fittings
4. How to check the thread size
An inch pipe-thread size is a nominal label. A 1-inch BSP male thread measures about 33.25 mm across the outside of its thread, not 25.4 mm.
For an accessible, isolated fitting, use callipers across the outer thread crests, from the top of one thread ridge to the opposite ridge across the centre. Check more than one position for taper, then check the thread pitch and the manufacturer's marking. A ruler or photograph can help narrow the choice, but cannot always distinguish similar threads.
| Nominal size | Approx. outside diameter | Threads per inch |
|---|---|---|
| ½-inch | 20.96 mm | 14 |
| ¾-inch | 26.44 mm | 14 |
| 1-inch | 33.25 mm | 11 |
| 1¼-inch | 41.91 mm | 11 |
| 1½-inch | 47.80 mm | 11 |
| 2-inch | 59.61 mm | 11 |
Measuring a female thread? Do not compare the inside opening with the male outside-diameter table. Use the manufacturer's drawing, the marking on the part or details of the known matching fitting. Send us the measurement and a photograph if you are unsure.
For garden taps and hoses: hose size and tap-thread size are different measurements. For example, a male BSP thread measuring about 26.4 mm across the outside is nominally ¾-inch. This does not mean that the hose must have a ¾-inch bore. For an ordinary threaded garden-tap connector, also check the specified sealing washer. A quick-connect hose end is another connection again.
These are identification references, not inspection limits. A tapered thread's measurement changes along its length. Do not use this table to size ACME or NPT threads, hose bore or plain pipe.
Reference for dimensions, pitch and identification: Swagelok thread identification guide (PDF).
5. Use the seal designed for the connection
- G/BSPP connections: use the specified washer, O-ring or seat. Extra tape does not replace a missing face seal.
- R-to-Rp/Rc and NPT connections: use only the sealing product and tightening method approved for that fitting and material.
- O-ring-sealed ACME sprinkler connections: keep the correct O-ring clean and intact. Follow the manufacturer's instructions; do not add PTFE tape to make a mismatched connection fit.
Hunter's TTS-800 instructions, for example, specify an O-ring seal and prohibit PTFE tape, PVC cement and other thread sealant on that ACME connection. Other assemblies can have different tightening or back-off instructions, so use the instructions for your exact model.
Isolate and fully depressurise the system before loosening any fitting. Never force mismatched threads or attempt to identify a connection by trial under pressure.
Manufacturer reference: Hunter TTS-800 installation guidance.
Unsure? Ask us before ordering
Send the team:
- A clear photograph of the complete sprinkler or swing joint.
- A close-up of the connection, any O-rings and moulded markings.
- The model and part number, plus the thread measurement if accessible.
- Details of the two parts you want to join.
Email us your connection photographs
Attach the photographs in your email to office@jkirrigation.co.uk. For a call, use 0333 335 6394, or visit our contact page. The website contact form does not accept photo attachments.
Related guides: Choosing poly pipe · Metric or imperial PVC · Water tank fittings
Technical guidance from UK Flow Centre, operated by JK Irrigation Ltd. Updated 13 September 2026.