BSPT vs. BSPP: Guide to Selecting Hydraulic Fittings

Selecting the wrong thread type for hydraulic fittings can lead to system leaks, typically due to errors regarding thread standards, taper, pitch, or sealing methods.

Here, I will clarify the often-confusing BSPT and BSPP standards. By examining four key aspects—sealing principles, visual identification, pressure ratings, and interchangeability—we will fully understand the differences between them, and I will provide a selection guide.

In my 20 years of professional experience, I have encountered numerous cases where an entire production line was brought to a standstill or new equipment could not be commissioned simply because a fitting's threads could not be screwed on, were installed incorrectly, or—even when installed—resulted in persistent oil leaks.

Let's start with a case study. A few years ago, we served a large mining equipment manufacturer. They purchased a core hydraulic system from Europe. During installation and commissioning, after all the pipelines were connected, multiple joints began to leak oil upon pressurization. The on-site engineers tried Teflon tape, various sealants, and even forcefully tightening the screws, but the problem persisted. The project was halted for three days, with daily losses in labor and project delays exceeding £10,000.

The original equipment interface uses a BSPP (British Standard Pipe) cylindrical pipe thread seal (G1/2"), with the end face sealed by a flat seal plus an O-ring or combination washer. The on-site installers, relying on "experience," assumed the interface thread outer diameter and pitch (14 threads per inch) matched the connector they had, and the interface end face looked "similar," so they purchased a BSPT (British Standard Tapered Pipe Thread Connector) (R1/2") for installation.

Both threads have a 55° thread angle and identical pitch, and the threads can be screwed in normally. They were unaware of the fundamental difference between a tapered seal and an end face seal: one relies on the thread taper itself for metal-to-metal sealing, while the other relies on the end face pressing against an O-ring for sealing.

The result was predictable—it leaked immediately during pressure testing, and even wrapping it with Teflon tape couldn't stop the leak, nearly cracking the female thread on the equipment.

What is BSP thread?

British Standard Pipe (BSP) thread is part of the imperial pipe thread system; it originated from Whitworth thread (BS 84) and features a 55° thread angle.

There are two forms of British Standard Pipe Threads that are used in the world today. They are BSPP (British Standard Pipe Parallel) and BSPT (British Standard Pipe Tapered). The BSPT male thread mates with the female BSPT thread, similar to an NPTF connection. The 30° BSPP male adapters connect to a female BSPP thread with a 30 ° cone. This style is comparable to an NPSM swivel style. These threads are almost identical to the NPTF pipe thread except for the flank angle. This angle is 55° versus 60° on the NPTF.

The current standard for BSPP (British Standard Parallel Pipe thread) is BS EN ISO 228-1; the current standard for BSPT (British Standard Taper Pipe thread) is BS EN 10226-1.

Tap water and gas piping systems in the UK and some Commonwealth countries still retain the traditional BSP thread design, which is particularly common in renovations of older buildings. BSP thread sizes—such as 1/8", 1/4", and 3/8"—refer to the pipe's "nominal size" rather than the actual thread diameter. In actual measurement, the outer diameter of a 1/4" BSP thread is approximately 13.16 mm; while close to the metric M14 thread, it is not identical.

BSPP&BSPT Thread Chart

BSPP Thread

BSPP is a parallel thread with a 55° thread angle. As the threads are parallel, they do not provide a seal on their own; sealing relies on a face seal (such as a bonded washer or O-ring). The advantages of BSPP threads include their suitability for a wide range of materials, such as steel, cast iron, and brass. They offer low manufacturing and operational costs, feature simple connection methods, and are easy to maintain. Additionally, BSPP threads provide excellent sealing performance, reliability, and stability, making them widely used in piping systems and mechanical equipment.

Sealing Method

Since the threads themselves are parallel, sealing cannot be achieved through radial compression generated by tightening.

Therefore, BSPP threaded connections must rely on additional sealing elements to prevent leakage. Common methods include using sealing washers or O-rings, or wrapping the threads with PTFE tape or applying thread sealant.

Applications

The design of BSPP threads makes them well-suited for connections requiring frequent assembly and disassembly. Widely used in equipment from Europe and Commonwealth nations, they are commonly found in low-pressure or static pipeline connections within hydraulic and pneumatic systems.

BSPT threads

BSPT features tapered threads with a 55° thread angle and a 1:16 taper; it achieves a seal through the deformation of the thread profile. Typically requiring the use of a sealant (such as PTFE tape or thread-locking compound), it is generally considered to provide a superior seal.

Sealing Method

The sealing principle of BSPT relies on a metal-to-metal connection between the tapered thread surfaces. As the threads are tightened, the tapered threads wedge against each other, generating high contact pressure to achieve a seal. Theoretically, it can achieve a seal without the use of any sealant or packing material.

Applications

Metal-to-metal contact ensures excellent sealing for BSPT threads. Additionally, tapered threads offer outstanding pressure resistance, effectively meeting sealing requirements for fluids and gases. Consequently, they play a vital role in piping systems transporting liquids, gases, steam, and hydraulic fluids, providing a secure and reliable seal. Due to a design that offers superior sealing performance compared to BSPP, BSPT threads are widely used in high-pressure environments, such as hydraulic systems.

Do not confuse BSPT with NPT. Although US-standard NPT threads also feature a 1:16 taper, their thread angle is 60°, making them incompatible with the 55° angle of BSPT threads. Forcing a mix of the two can damage the threads and cause seal failure; always use the appropriate thread gauges for verification. These represent two distinct standard systems—Imperial/British and US—with different regional applications (BSP is common in Europe and Asia, while NPT is primarily used in North America).

How to distinguish between BSPP and BSPT

Identifying BSP threads starts with determining if it is a parallel or tapered thread. Next, measure the lead thread diameter. Compare this measurement to the listed dimensions to determine size. If instruments are not available to measure this, you can compare it end-to-end with a known NPTF thread to approximately arrive at the nominal BSP size. Finally, measure the pitch and compare it to the size chart to complete the identification. These dimensions will be the same for both BSPP and BSPT.

How to choose between BSPP and BSPT

Pressure: Selection can be based on the pressure environment; parallel threads (BSPP) with a sealing ring are suitable for low-pressure or static systems, while tapered threads (BSPT) may be considered for high-pressure environments.

Sealing: BSPT is often selected for applications requiring reliable sealing under high ambient pressure, whereas BSPPP offers greater operational convenience for pipe connections that require frequent disassembly and maintenance.

In general, BSP threads are commonly used in low-pressure piping systems; among them, tapered threads (BSPT) achieve a seal through thread deformation and typically offer superior sealing performance, whereas parallel threads (BSPP) are better suited for connections requiring frequent assembly and disassembly.

Proper Maintenance of BSP Fittings

The maintenance of British Standard Pipe (BSP) fittings centers primarily on their sealing performance, with specific maintenance requirements varying according to the thread type (BSPP vs. BSPT).

BSPP (parallel pipe threads) are cylindrical threads that do not provide a seal on their own; they rely on end-face seals—such as bonded washers or O-rings—to ensure a leak-proof connection.

BSPT (tapered pipe threads) rely on the deformation of the thread profile to create a seal, though the use of a sealant is typically required to ensure integrity. Without sealant, leakage paths may remain between the threads, leading to leaks.

Maintenance strategies should be tailored to the specific application: BSPT tapered threads offer a tighter, more secure seal, whereas BSPP parallel threads are better suited for fittings that require frequent assembly and disassembly. Tapered threads (such as BSPT) are preferable for high-pressure environments, while parallel threads (such as BSPP) paired with a sealing ring are sufficient for low-pressure or static systems.

FAQ

How do I choose between BSPP and BSPT?

Choose BSPP: When the connection design incorporates a face seal (such as an O-ring groove or a flat sealing surface), system pressure is low, or frequent disassembly for maintenance is required.

Choose BSPT: When the connection must withstand higher pressures, handle hazardous media, or requires a more reliable metal-to-metal seal. It is the standard choice for many hydraulic systems.

How to seal BSPP fittings?

For BSPP threads, you should ensure the mating face is flat and clean and select an appropriate gasket or O-ring. If using thread seal tape or sealant, cover the entire thread length but avoid applying an excessive amount to prevent material from being squeezed into the pipeline.

How to seal BSPT fittings?

The primary seal for BSPT fittings relies on the mating of tapered surfaces. When applying PTFE tape, 2–3 layers are typically sufficient; the tape should be wrapped in the direction opposite to the tightening rotation to prevent the end from being pulled into the threads. Sealant should be applied evenly to avoid clogging small-bore piping.

What is the difference between BSPT and NPT?

Although American NPT threads also feature a 1:16 taper, they have a 60° thread angle, making them incompatible with the 55° angle of BSPT threads. Forcing a mix of the two can damage the threads and lead to seal failure. Always use the appropriate thread gauges for verification.