1/8 BSP Thread – what kind of thread is it

The 1/8 BSP (British Standard Pipe) thread is a precision pipe thread belonging to the group of the most commonly used connections in power hydraulics, pneumatics, and industrial installations. It features 28 threads per inch (28 TPI) and an outer diameter of approximately 9.7 mm, with an inner diameter within the range of 8.5–8.9 mm. Due to its small dimensions and high tightness, this thread is used in measuring and auxiliary systems where compact design and reliability matter.

What makes the 1/8 BSP thread stand out?

BSP threads with a 55° profile come in two basic varieties:

• BSPP (Parallel) – a parallel thread, sealed using copper or aluminum washers,

• BSPT (Tapered) – a tapered thread that self-seals due to the convergence of the threads.

This design ensures a durable and vibration-resistant connection, which is particularly important in hydraulic systems exposed to variable operating pressure. The 1/8 BSP thread is commonly used in components such as couplings, elbows, tees, nipples, fittings, and plugs used in measuring and lubricating devices.

1/8" thread how many mm

1/8 thread how many mm – this is one of the most frequently asked questions when selecting hydraulic fittings, pressure gauges or valves. The 1/8 inch BSP thread has an outer diameter of approximately 9.7 mm and 28 threads per inch (28 TPI). However, this does not mean that 1/8 inch = 3.175 mm – in pipe threads, the inch designation does not correspond directly to the diameter measured with a caliper. In Hydron's offer a full range of BSP inch threads is available, including 1/8 BSP thread used in power hydraulics, pneumatics, and industrial installations.

1/8 thread how many mm – exact conversion

A common error involves a simple conversion:

1 inch = 25.4 mm
1/8 inch = 3.175 mm

This conversion is mathematically correct but does not apply to BSP pipe threads. In pipe threads, the inch designation is a trade name, not a direct diameter.

Application of the 1/8 BSP thread

Due to its dimensions and properties, the 1/8 BSP thread is widely used in:

• connections for pressure gauges, pressure and temperature sensors,

• centralized lubrication systems,

• low-pressure pneumatic and hydraulic installations,

• measuring and control lines,

• industrial and laboratory equipment.

This thread is also frequently used in test systems where high tightness and quick assembly and disassembly of the connection are required.

Why choose BSP threads available at Hydron?

Hydron offers a wide selection of fittings and components with BSP threads, made of the highest quality materials resistant to corrosion and mechanical damage. The products available in our store meet industry requirements and quality standards, guaranteeing reliability and operational safety even in difficult industrial conditions. Our team of experts assists in selecting the appropriate fittings and adapters to ensure full compatibility with existing hydraulic and pneumatic systems.

Technical parameters of the 1/8 BSP thread:

• Thread type: BSP (British Standard Pipe)

• Thread pitch: 28 threads/inch

• Outer diameter: approx. 9.7 mm

• Inner diameter: 8.5–8.9 mm

• Profile angle: 55°

• Versions: BSPP (parallel), BSPT (tapered)

• Typical applications: pressure gauges, sensors, measuring systems, lubrication installations

Order BSP threads at the Hydron store and provide your installations with durable, leak-proof, and fully safe hydraulic connections.

Differences between 1/8 BSP and 1/8 NPT

One of the most common assembly problems is confusing 1/8 BSP with 1/8 NPT. Although the diameters are similar, these standards differ in the thread profile angle and the sealing principle. The BSP thread has a 55-degree angle, while NPT has a 60-degree angle. The NPT version is always tapered and seals on the thread, whereas the cylindrical BSP version requires an additional sealing element. Attempting to connect these standards can result in thread damage or a permanent leak. In power hydraulics, there is no room for compromise in this regard, which is why identifying the thread type is an absolute basic requirement for correct assembly.

Most common errors in 1/8 thread identification

In service practice, it often happens that a user measures approximately 9–10 mm and assumes they are dealing with an M10 metric thread. However, metric threads have a different pitch and profile angle. Another error is ignoring the number of threads per inch and the type of seal. Without this information, the standard cannot be clearly determined. At Hydron, the selection of threaded components is based on ISO standards and years of experience in power hydraulics. In case of doubt, identification based on a photo or measurement is possible, which helps avoid machine downtime.

FAQ

What is the diameter of a 1/8 BSP thread in millimeters?
A 1/8 BSP thread has an outer diameter of approximately 9.7 mm and 28 threads per inch.

Is 1/8 inch equal to 3.175 mm in the case of a thread?
No. This mathematical conversion applies to length, whereas in pipe threads, the 1/8 designation does not directly correspond to the thread diameter.

Are 1/8 BSP and 1/8 NPT interchangeable?
They are not interchangeable. They differ in the profile angle and the sealing principle.

Where is the 1/8 BSP thread used?
Most commonly in power hydraulics, pneumatics, pressure gauges, and pressure sensors.

How much do components with a 1/8 BSP thread cost?
The price depends on the type of component, such as a fitting, plug, or reduction. Current prices are available next to the products in the Hydron store.