A thread on a molded-in brass insert cannot be reworked once the plastic is around it, so the standard, the sealing method and the gauge have to be right on the drawing before the first shot.
Short answer: BSP and NPT threads are not interchangeable on a brass insert. BSP follows the ISO 7-1 / EN 10226-1 family (sealing on the thread) or ISO 228-1 (parallel, sealing on a gasket or face), while NPT follows ASME B1.20.1. Pick the standard used by the destination market and the mating part, then write the exact designation, sealing method and gauge standard on the drawing.
Key Takeaways
- “BSP” is not one thread: sealing BSP (ISO 7-1 / EN 10226-1) and parallel BSP (ISO 228-1) seal in completely different places.
- ISO 228-1 threads are parallel on both sides and are not made pressure-tight on the thread; sealing happens between two faces outside the thread.
- ISO 7-1 and EN 10226-1 threads seal on the thread flanks, and a sealant or jointing compound is expected on the thread.
- NPT is defined in ASME B1.20.1 with a 60° thread form, so it does not mate reliably with 55° BSP threads even where the pitch looks close.
- On an RFQ, write the standard number, designation, internal or external, gauge standard and mating part, not just “1/2 inch BSP.”
What is the difference between BSP and NPT threads on a brass insert?
The difference between BSP and NPT threads is the defining standard, and that standard fixes the thread angle, the profile and the sealing logic. BSP threads come from the British Standard Pipe family now published as ISO and EN standards, while NPT threads come from the American National Pipe Taper family in ASME B1.20.1. An insert machined to one will not form a dependable joint with a fitting machined to the other.
The thread form is the most visible split. BSP threads use the 55° Whitworth flank angle, and NPT threads use a 60° flank angle with flattened crests and roots. Several common sizes have similar or identical threads per inch, which is why mismatched parts can engage for a turn or two before they bind or leak.
The sealing logic is the less visible split, and it matters more for insert design. Some pipe threads are designed to seal on the thread flanks themselves; others are only fastening threads and need a separate seal. That distinction decides the shape of the insert face, the length of the thread and whether the fitting needs a gasket seat.
Brass inserts carry these threads because the plastic body of a PPR, PE, CPVC or PPSU fitting cannot hold a metal-to-metal threaded joint on its own. The insert is the part that meets the threaded valve, tap or pipe, so its thread standard has to match whatever the installer will screw into it. Our overview of brass insert fittings shows the typical male and female insert forms.
Which threads seal on the thread, and which need a gasket?
Threads from ISO 7-1, EN 10226-1 and ASME B1.20.1 NPT seal on the thread, while ISO 228-1 threads never seal on the thread and need a gasket, O-ring or face seal. The standards state this directly in their scopes, so it is not a matter of installer preference.
ISO 7-1 covers pipe threads where pressure-tight joints are made on the threads, in the combinations taper external with parallel internal or taper internal. Its scope also notes that parallel external pipe threads are not suitable as jointing threads, and it recommends a jointing medium on the thread.
BS EN 10226-1:2004 is the European standard for the taper external, parallel internal combination in sizes 1/16 to 6. Its scope says an appropriate thread sealant or jointing compound should be used on the thread to ensure pressure-tight joints.
EN ISO 228-1 covers pipe threads where pressure-tight joints are not made on the threads, also in sizes 1/16 to 6. Both internal and external threads are parallel, and where a tight joint is needed the standard says it should be made by compressing two tightening surfaces outside the threads with an appropriate seal between them.
| Thread family | Defining standard | Common designation | Thread geometry | Where it seals | Typical insert feature needed |
|---|---|---|---|---|---|
| Sealing BSP, taper/parallel | ISO 7-1, EN 10226-1 | R (taper external), Rp (parallel internal) | 55° form; taper male, parallel female | On the thread, with sealant | Adequate engaged length; no gasket seat |
| Sealing BSP, taper/taper | ISO 7-1 | R (taper external), Rc (taper internal) | 55° form; taper both sides | On the thread, with sealant | Tapered internal thread machined after forging |
| Parallel BSP | ISO 228-1 | G | 55° form; parallel both sides | On a gasket, O-ring or face outside the thread | Flat sealing face or groove; thread relief |
| NPT | ASME B1.20.1 | NPT | 60° form; taper both sides | On the thread, with sealant | Engaged length matched to taper |
| Straight pipe, mechanical | ASME B1.20.1 | NPSM, NPSL and related | 60° form; parallel | Not on the thread | Seat or gasket for sealing |
The thread types listed for ASME B1.20.1 follow the current 2013 (R2018) edition, which covers dimensions and gaging of NPT, NPSC, NPTR, NPSM and NPSL threads. The table summarizes sealing intent and geometry; it does not replace the dimension tables in the standards themselves.
Can BSP and NPT threads be mixed on the same fitting?
BSP and NPT threads should not be mixed on the same joint, because the 55° and 60° flank angles never seat fully against each other. A mixed joint may hand-tighten and even pass a short test, but the flanks contact only along part of their height, so the seal depends on extra sealant filling a gap that the standards did not intend.
Mixing sealing and non-sealing BSP is a second, quieter mistake. A taper R male thread screwed into a parallel G female thread can engage, but G threads are fastening threads under ISO 228-1, so the joint has no defined sealing mechanism. The reverse, a parallel G male into an Rp or Rc female, falls under ISO 7-1’s warning that parallel external threads are not suitable as jointing threads.
A fitting can still carry two different standards on different ports. An elbow may have a parallel G thread on one end that seals on a gasket against a meter and a taper thread on another end that seals on the thread. Each port’s designation simply has to appear on the drawing, and each insert has to be machined and gauged to its own standard.
Our earlier article on brass insert failure modes covers leakage at the insert–plastic interface. Thread mismatch is a different leak path: it leaks at the threaded joint even when the insert is perfectly bonded into the fitting.
Where do metric threads fit in?
Metric threads fit in as fastening and connector threads, not as pipe threads, so they appear on inserts mainly for union nuts, gauges and equipment-specific connections. ISO 261 specifies ISO general purpose metric screw threads (M) with the basic profile of ISO 68-1, with basic dimensions in ISO 724 and tolerances in ISO 965-1.
An M thread is designated by diameter and pitch, for example M20 × 1.5, and that designation is precise in a way that “1/2 inch” is not. If a customer’s equipment uses metric connections, the drawing should give the full M designation and the tolerance class rather than an approximate pipe size.
Pipe sizes such as 1/2 or 3/4 are nominal names, not measured diameters. The real outside diameter of a 1/2 BSP thread is well over half an inch, which is a common source of confusion when buyers measure a sample with calipers. Fenilo’s pipe thread size converter maps nominal BSP and NPT sizes to their measured diameters.
How does the thread choice change the insert design?
The thread choice changes the insert’s sealing face, thread length and wall thickness, which in turn changes the forging blank and the molded fitting around it. A thread standard is therefore an input to insert design, not a detail added at the end.
A parallel G thread needs somewhere to seal. That usually means a flat annular face for a gasket, a groove for an O-ring, or a shoulder that a union nut can clamp against. The face has to stay outside the plastic and be machined flat after forging, so it adds length and a finishing operation.
A taper thread seals on its flanks, so it needs enough engaged length to build up interference as it tightens. It does not need a gasket face, but the wall behind the thread must resist the radial load a taper generates when the installer applies torque. On a female insert, that load is transmitted into the surrounding plastic.
The knurl and anti-rotation features have to match the thread type as well. A taper joint is tightened against rising resistance, so the insert must resist the full installation torque without spinning in the plastic. Our guide to insert molding knurl design explains how knurl pattern, shoulders and flats carry that torque.
How should you write the thread specification on an RFQ?
Write the thread specification as a complete designation tied to a published standard, plus the sealing method, gauge standard and mating part. A line such as “1/2 inch BSP” leaves the supplier to guess between R, Rp, Rc and G, and each guess produces a different insert.
| RFQ field | What to write | Example |
|---|---|---|
| Standard number and edition | The standard the thread is made to | ISO 228-1; EN 10226-1; ASME B1.20.1 |
| Designation | Full thread code and size | G 1/2; Rp 3/4; 1/2-14 NPT; M20 × 1.5 |
| Internal or external | Which side the insert carries | Female (internal) thread |
| Sealing method | Thread sealant, gasket, O-ring or face seal | Flat gasket on face Ø specified |
| Gauge standard | How the thread will be accepted | Gauged per ISO 228-2 or per the standard’s gauging section |
| Thread length and relief | Usable length and runout limits | Full thread length per drawing |
| Mating part | What screws into the insert | Valve body, meter, tap connector |
| Destination market | Where the fitting will be sold | EU, Middle East, North America |
| Host resin | Plastic of the molded fitting | PPR, PE, CPVC or PPSU |
The gauge line deserves particular attention. The SIS listing for ISO 228-1 notes that ISO 228-2 gives methods for verifying fastening thread dimensions and form and recommended gauging systems, and taper threads are likewise accepted with gauges defined for their standard. Agreeing the gauge in advance prevents a dispute in which both sides measured correctly against different references.
The destination market usually decides the standard. Taper and parallel BSP threads are the norm across much of Europe, the Middle East, Asia and Australia, while NPT dominates North American plumbing. If the fitting is going to more than one market, quoting both thread versions as separate part numbers is cleaner than looking for a compromise thread.
How does Fenilo handle thread specifications on custom inserts?
Fenilo machines the thread on each brass insert by CNC after hot forging, so the thread standard on the customer’s drawing is cut into the part rather than formed in the die. The production route runs brass rod, hot forging, sandblasting, CNC machining, assembly, testing and packaging in one plant in Yuhuan, Zhejiang, which keeps the forging blank and the thread machining under one roof.
Threads are checked at two of Fenilo’s five quality-control stages. In-process dimensional inspection verifies dimensions during machining, and finished-goods inspection covers appearance, threads, assembly and packaging before shipment. Incoming inspection of all brass material and a continuous-improvement loop based on production data and customer feedback complete the system, with pressure and leak testing applied to valve products before they ship.
The insert body is designed around the customer’s molding process as well as the thread. Fenilo hot forges CW617N and HPb59-1 brass, customizes knurl pattern and dimensions to the customer’s injection molding process, and supplies inserts for overmolding in PPR, PE, CPVC and PPSU fittings. Stand-alone brass thread fittings are part of the same product range.
Samples can be supplied for assembly trials and quality evaluation, which is the practical way to confirm thread fit against the customer’s own mating parts and gauges. Tolerances, lead times and minimum order quantities are confirmed per order against the drawing, since minimum quantity depends on product category and the degree of customization.
Specifying threaded brass inserts for your fittings?
Fenilo hot forges CW617N and HPb59-1 brass inserts and CNC-machines the thread to your drawing in its own plant in Yuhuan, Zhejiang. Knurls are customized to your molding process, threads are checked during machining and again at finished-goods inspection, and samples are available for fit trials with your mating parts.
Fitting manufacturers setting up a new insert program can also start from our page for pipe and fitting manufacturers, which outlines how a custom insert moves from drawing to repeat supply.
FAQ
Is BSP the same as G thread?
BSP is not always the same as G thread, because BSP covers both sealing and non-sealing families. G is the designation for parallel threads under ISO 228-1, which do not seal on the thread. Sealing BSP threads are designated R, Rp or Rc under ISO 7-1 and EN 10226-1 and are made pressure-tight on the thread with a sealant.
Can an NPT male thread be screwed into a BSP brass insert?
An NPT male thread should not be screwed into a BSP brass insert, because the 60° NPT flank angle does not seat against the 55° BSP flank angle. The parts may engage for a few turns in sizes with similar pitch, but the joint relies on sealant to fill gaps and is not covered by either standard. Specify the insert to the same standard as its mating part.
Which BSP thread should a brass insert use for a gasket seal?
A brass insert sealed by a gasket should use a parallel G thread to ISO 228-1. That standard states that its threads are not made pressure-tight on the thread and that sealing should be effected by compressing two tightening surfaces outside the threads with a seal between them. The insert therefore needs a flat face or groove for the gasket.
Does a taper thread on a brass insert need thread sealant?
A taper thread on a brass insert does need thread sealant in normal practice. BS EN 10226-1 states that an appropriate thread sealant or jointing compound should be used on the thread to ensure pressure-tight joints, and ISO 7-1 likewise recommends a jointing medium. The sealant fills the clearance between flanks that the taper alone does not close.
How do I write the thread on a brass insert drawing?
Write the thread on a brass insert drawing as a full designation with its standard, for example “G 1/2 to ISO 228-1, internal” or “1/2-14 NPT to ASME B1.20.1, external.” Add the sealing method, the gauge standard used for acceptance and the mating part. A nominal size alone, such as “1/2 inch BSP,” leaves the thread family undefined.
Bottom line: a brass insert’s thread is fixed for the life of the molded fitting, so treat the thread specification as a design decision, not a catalog checkbox. Match the standard to the destination market and the mating part, decide whether the joint seals on the thread or on a gasket, and put the full designation, sealing method and gauge standard on the drawing before tooling starts.