BA - British Association
A thread design first proposed in 1884 and then adopted in 1903.
The thread sizes are designated by a number from 0 to 22.
0BA is 6mm in diameter and has a 1.00mm thread pitch.
The sizes then progress with each larger number screw having a pitch of 0.9 times the pitch of the previous size. (0.9p) The result is rounded to two significant figures in metric and then converted to imperial and rounded to thousandths of an inch.
The major diameter is 6 x the pitch.
The hex size is 1.75 x the major diameter
The pitch diameter of the thread is simply OD − 0.6p
The minor diameter is OD − 1.2p.
The thread angle is based on the Swiss Thury thread, but is not quite the same. BA screws use a thread angle of 47.5° exactly and the thread form to be symmetrical with a depth of 3/5p.
The British Standards Institution recommended the use of BA sizes in favor of the smaller Britsh Standard Whitworth (BSW) and British Standard Fine (BSF) thread sizes below 1/4".
BA threads found common use in electrical goods made in the UK and its colonies. It can also be found connecting the barrel of a dart to its shaft.
Very popular still with model engineers who find the head to screw diameter relationship desirable.