British Association Threads (BA)

BA Threads

This thread was used for small diameter threads (below 0.25 inches diameter). The thread has radiused roots and crests and has a flank angle of 47½°. The thread size varies from BA number 23 (0.33 mm diameter with a pitch of 0.09 mm) to BA number 0 (6mm diameter with a pitch of 1 mm). Relative to the Whitworth thread the depth of the BA thread is smaller size for size. The thread form is now redundant and has been replaced by Unified and Metric threads. The form of the thread is shown in the diagram:

British Association threads were a metric thread system devised for small screws. Originally included even smaller sizes intended for use in watchmaking but these have been superceded by specialist horological thread series. Not metric like you might expect, but with diameters determined by a factor proportional to a power of the logarithm to the base 10 of the thread pitch in millimetres. Hence the rather strange mix of metric and imperial in the above table.

BA Thread Form

Model Engineer Thread (ME)

ME Threads

The model engineer threads are based on the Whitworth thread and are fixed pitch in two ranges 32 and 40 tpi. Some model boiler fittings use these threads but otherwise it is not used commercially and there are no nuts bolts etc. made to this standard.

BA Threads