Information on Screw Threads

British Association Thread

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 47 degrees and a half degrees. 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 The thread form is now redundant and has been replaced by Unified and Metric threads.The form of the thread is shown in the diagram:


p = pitch of the thread

d = depth of the thread

r = radius at the top and bottom of the threads


d = 0.6 p

r = 2 p /11
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