BSP thread: what it is and where it occurs

The phrase what is a BSP thread very often appears among people servicing hydraulic and pneumatic systems. BSP thread is an inch pipe thread commonly used in Europe, especially in power hydraulics, pneumatics and oil installations. The most common variety is BSPP, i.e. a cylindrical thread with a 55° profile angle, which in itself does not ensure tightness. Sealing is achieved through a washer, o-ring or flat seal in the socket. BSP threads are found in fittings, pressure gauges, valves and manifolds. Example dimensions and descriptions of specific sizes are available on pages such as

How to measure an inch thread

Measuring an inch thread is one of the most common activities performed when selecting hydraulic, pneumatic or fuel components. Correct determination of the thread type and size is crucial for the tightness and safety of the connection. Below you will find a simple way to measure an inch thread to avoid mistakes when selecting fittings and ends.

What is the difference between an inch thread and a metric one ?

An inch thread (marked e.g. G, BSP, BSPT, NPT) differs from a metric one primarily in:

• measurement unit – instead of millimeters, inches are used;

• profile angle – inch threads have a 55° angle (BSP) or 60° (NPT), while metric ones have 60°;

• designation method – e.g. 1/4”, 3/8”, 1/2”, 3/4”, 1” etc.

BSP thread dimensions ?

To measure the diameter use calipers and measure the external diameter of the thread (for external thread) or internal (for internal thread). Example approximate diameter values for the most common BSP threads:

• 1/8" – approx. 9,7 mm,

• 1/4" – approx. 13,2 mm,

• 3/8" – approx. 16,7 mm,

• 1/2" – approx. 20,9 mm,

• 3/4" – approx. 26,4 mm,

• 1" – approx. 33,2 mm.

Note: these values do not directly correspond to the dimension in inches, because the inch size refers to the internal diameter of the pipe, and not to the external diameter of the thread.

How to determine thread pitch (number of threads per inch)?

Inch threads are measured not in millimeters, but in the number of threads per inch (TPI – Threads Per Inch). To determine TPI:

• use an inch thread pitch gauge,

• or count the number of threads over a length of 1 inch (25,4 mm).

For example:

• 1/4” BSP thread has 19 threads per inch,

• 1/2” BSP thread has 14 threads per inch,

• 3/4” BSP thread has 14 threads per inch.

How to recognize the type of inch thread ?

The most common types of inch threads are:

• BSPP (G) – parallel thread, used in connections with a flat seal (e.g. copper washer).

• BSPT (R) – tapered thread, seals on the thread profile itself.

• NPT / NPTF – American tapered thread, with a 60° angle, differing in geometry from BSP.

To distinguish BSP from NPT, pay attention to the shape: the NPT thread has a clear narrowing towards the end and is sharper.

Practical measurement tips:

If you are unsure, always compare the connection with a catalog or use an inch thread gauge.  In power hydraulics and pneumatics, inch threads occur very often, especially in fittings of the BSP (G/R) type. At the Hydron store, you will find detailed descriptions and technical drawings when buying fittings that will help you choose the right type of thread. To correctly measure an inch thread, you should:

1. Measure the diameter with calipers.

2. Determine the number of threads per inch (TPI).

3. Check if the thread is parallel (BSPP) or tapered (BSPT).

4. Compare the result with the inch thread table.

In the Hydron store you will find a wide selection of fittings, ends and components with inch threads, and our specialists will help you choose the right type and size of connection.

BSP vs BSPT thread – how they differ and when to use which

Comparing BSP and BSPT threads is one of the most common issues arising when selecting hydraulic fittings. Both standards derive from the British Standard Pipe norm, however, they differ in construction and sealing method. BSP (BSPP) thread is a cylindrical thread and requires sealing on the face surface, most often using a washer or o-ring. In turn, the BSPT thread is a tapered thread that seals directly on the threads, similar to NPT. In practice, this means that BSP and BSPT threads are not interchangeable without knowledge of the socket design and sealing method.

BSPP thread – what it means and how it differs from BSPT

The phrase BSPP thread refers to the cylindrical variety of the British Standard Pipe thread, where the abbreviation BSPP stands for a straight (parallel) thread. Unlike the BSPT thread, the BSPP thread is not a thread that seals on the threads. Connection tightness is achieved through a face seal, most often using a metal-rubber washer or an o-ring seated in the socket. BSPP thread is very often used in power hydraulics and pneumatics, especially in pressure gauges, valves, quick couplers and service fittings, where the possibility of repeated assembly without damaging the thread is important.

Rp thread vs BSPP – construction differences and application

Rp thread and BSPP thread differ primarily in geometry and the method of sealing the connection, which has a direct impact on their application. Rp thread is an external tapered thread, which seals on the thread during tightening. The tapered construction causes the threads to wedge, making it possible to achieve tightness without sealing on the face of the connection. In practice, PTFE tape or sealing paste is often additionally used. In turn, the BSPP thread is a cylindrical thread, which in itself is not responsible for the tightness of the connection. In this case, tightness is achieved on the face, using an o-ring, a metal-rubber washer or a flat gasket. Thanks to this, BSPP connections are characterized by high assembly repeatability, resistance to vibration and the possibility of multiple disassembly without the risk of damaging the thread. R and BSPP threads are not interchangeable, despite the identical thread profile angle and similar size markings.

BSP vs NPT thread – what they are and why they are confused

The phrase BSP vs NPT thread very often appears when selecting fittings hydraulic, because both standards are inch threads and occur in similar nominal diameters. Despite visual similarities, they are two different thread systems that are not compatible with each other without using an adapter. Incorrectly distinguishing between BSP and NPT thread most often leads to leakage, thread damage or problems with the assembly of power hydraulic components.

NPT vs BSP thread technical differences

The most important difference between BSP thread and NPT is the thread geometry. BSP has a 55° angle, while NPT has 60°, which causes the threads to not fit together correctly. Additionally, the thread pitch and the sealing method differ. In practice, this means that even if the elements can be partially screwed in, the connection will not be tight and safe. Attempts at forced assembly often end in damage to the socket or thread.

BSP thread table : 

Below are the approximate external dimensions of the most commonly used BSP (G) inch threads found in hydraulic and pneumatic fittings: