British Association BS 93

The British Association thread is a British thread with very small dimensions, widely used in the watch industry, precision engineering, and model making, and still used there today, albeit to a lesser extent. The origins of the British Association thread date back to 1884 when the design was first introduced. Since then, the thread has been revised and is now used according to the measurements of the British Standards norm BS 93. Its use is mainly limited to Great Britain, where the British Standards Institution (BSI) recommends using the British Association thread for threads under 1/4" instead of the BSW or BSF threads. In the USA, the British Association thread has a very specific application: it is used to connect the individual parts of a dart.

Unlike most other British thread types, the British Association thread is a metric thread. The flank angle is 47.5°, distinguishing it not only from Whitworth threads but also from the metric thread.

The thread designation contains no measurements of the British Association thread. Instead, the individual thread sizes are numbered, starting at 0 and ending at 25. Preceded only by the abbreviation BA, a possible thread designation for the British Association thread, for example, would be BA No. 3.

In the thread table for the British Association thread, it's important to note that the thread sizes are sorted in descending order. BA No. 0 is not the smallest but, with a 6 mm external diameter, the largest possible variant for this thread. The measurements of the British Association thread, such as external diameter, core hole diameter, and pitch, are all given in mm. We have dies and taps for the most common sizes of the British Association thread in stock, which you can access via the corresponding links. Should you need threading tools for the British Association thread beyond these, we are happy to manufacture them as a custom order!