BSB Thread Pitch Chart

What is a BSB Thread?

BSB stands for British Standard Brass. BSB threads are a British thread standard originally developed for brass components and applications where fine threads were required. Based on the Whitworth thread form, BSB threads use a distinctive 55° thread angle that sets them apart from modern metric and imperial standards.

BSB threads were commonly used in brass fittings, valves, instruments and specialist mechanical components throughout the 20th century. Although less common in modern manufacturing, BSB threads can still be found in older equipment, restoration projects and specialist engineering applications where original specifications must be maintained.

Understanding BSB Thread Sizes

BSB threads are identified by their nominal size and threads per inch (TPI). The thread form is similar to other British Whitworth-based thread standards but has specific dimensions suited to brass components. Each BSB thread size has a defined major diameter, pitch and corresponding tap drill size.

Correct identification is critical because BSB threads are not interchangeable with metric, UNC, UNF or other imperial thread systems. Using the wrong thread standard can result in poor fit, stripped threads or component failure. The BSB Thread Chart above provides the essential dimensions needed to identify and work with these threads accurately.

How to Read the BSB Thread Chart

The chart above displays the key specifications for each BSB thread size:

• Diameter – The nominal size of the thread in imperial measurements (for example, 5/16", 3/8", 1/2")
• TPI (Threads Per Inch) – The number of thread peaks per inch, which defines the thread pitch
• Drill Size (mm) – The recommended tap drill diameter in millimetres, used when preparing a hole for tapping

To use the chart, locate your BSB thread size in the left column, then reference the corresponding TPI and drill size. This information is essential when selecting the correct BSB tap or preparing a hole for threading.

Common Applications

BSB threads are found across a wide range of specialist applications:

• Brass fittings and connectors
• Valves and plumbing components
• Vintage machinery and equipment
• Restoration projects and heritage engineering
• Model engineering and precision work
• Musical instruments and precision mechanical components
• Older British manufactured equipment
• Specialist industrial and laboratory instruments

If you're working with equipment manufactured in Britain before the 1970s, or restoring vintage machinery, BSB threads are likely to be present in brass fittings and components.

BSB vs Other British Thread Standards

Several British thread standards exist, and it's important to understand the differences:

BSB (British Standard Brass) – Designed specifically for brass components, based on the Whitworth thread form with a 55° thread angle. Used in brass fittings, valves and specialist applications.

BSW (British Standard Whitworth) – The general-purpose British coarse thread standard, used for larger mechanical fasteners and general engineering applications. BSW threads have a 55° angle but different pitch specifications than BSB.

BSF (British Standard Fine) – The fine version of Whitworth threads, offering tighter pitch for applications requiring finer adjustment or higher strength.

Although these thread standards may look similar, they should not be mixed without confirming dimensions. Using the correct thread standard ensures proper fit, function and safety in your application.

Frequently Asked Questions

What does BSB stand for?
BSB stands for British Standard Brass. It is a British thread standard originally developed for brass components and applications requiring fine threads.

What angle is a BSB thread?
BSB threads use a 55° thread angle, which is characteristic of Whitworth-based British thread standards.

Are BSB threads still used today?
While less common in modern manufacturing, BSB threads are still found in restoration projects, vintage equipment, specialist engineering applications and heritage machinery where original specifications must be maintained.

Are BSB threads the same as BSW threads?
No. Although both are based on the Whitworth thread form with a 55° angle, BSB and BSW have different pitch specifications and applications. BSB was designed for brass components, while BSW is a general-purpose coarse thread standard. They are not interchangeable.

Can BSB threads be replaced with metric threads?
BSB and metric threads have different dimensions and thread angles, so they are not directly interchangeable. If replacement is necessary, the component must be re-drilled and tapped to the new standard, which may not be practical for restoration work. Whenever possible, use the original BSB specification.