This thread has been around for a long time. It was first formulated in 1884 and was standardised in 1903. Mainly found in electrical fittings and accessories, it is slowly being replaced by ISO metric.

Angle of thread = 47. 5 degrees, rounded at top and bottom

BA Thread Profile

British Association (BA)

47-1/2 degree thread angle. This is a metric thread system devised by the British for small screws used in components like speedos. 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 millimeters.

You'll find lots of BA threads on any British car, but only for fasteners smaller than 1/4". BA fasteners have their own set of spanner sizes. Typically, a set of "Whitworth" sockets will include a OBA (and maybe a 2BA--bigger number = smaller size) socket.

Screw Numbers	Diameter of Screw Over Thread	Threads per inch	Pitch	Diameter at Thread Bottom	Tapping Drill Size
16 BA	0.0310 inch	133.3 tpi	0.0075 inch	0.0220 inch	Number drill 73 0.6 mm 0.0236 inch
15 BA	0.035 inch	120.5 tpi	0.0083 inch	0.025 inch	Number drill 70 0.7 mm 0.0276 inch
14 BA	0.039 inch	109.9 tpi	0.0091inch	0.028 inch	0.8 mm 0.0315 inch
13 BA	0.047 inch	102 tpi	0.0098 inch	0.0352 inch	Number drill 62 0.98 mm 0.0386 inch
12 BA	0.051 inch	90.9 tpi	0.011 inch	0.0378 inch	Number drill 60 1.05 mm 0.0413 inch
11 BA	0.059 inch	81.9 tpi	0.0122 inch	0.0443 inch	Number drill 56 1.2 mm 0.0472 inch
10 BA	0.067 inch	72.6 tpi	0.0138 inch	0.0504 inch	Number drill 54 1.4 mm 0.0551 inch
9 BA	0.075 inch	65.1 tpi	0.0154 inch	0.0565 inch	1.55 mm 0.061 inch
8 BA	0.086 inch	59.1 tpi	0.0169 inch	0.0657 inch	Number drill 50 1.8 mm 0.0709 inch
7 BA	0.0982 inch	52.9 tpi	0.0189 inch	0.0753 inch	Number drill 46 2.05 mm 0.0807 inch
6 BA	0.11 inch	47.9 tpi	0.0209 inch	0.0849 inch	2.3 mm 0.0906 inch
5 BA	0.126 inch	43 tpi	0.0232 inch	0.0981 inch	Number drill 37 2.65 mm 0.1043 inch
4 BA	0.1420 inch	38.5 tpi	0.0260 inch	0.1108 inch	Number drill 31 3 mm 0.1181 inch
3 BA	0.1610 inch	34.8 tpi	0.0287 inch	0.1266 inch	Number drill 29 3.4 mm 0.1339 inch
2 BA	0.1850 inch	31.4 tpi	0.0319 inch	0.1467 inch	Number drill 22 4 mm 0.1575 inch
1 BA	0.2090 inch	28.2 tpi	0.0354 inch	0.1665 inch	Number drill 16 4.5 mm 0.1772 inch
0 BA	0.2360 inch	25.4 tpi	0.0394 inch	0.1887 inch	Number drill 9 5.1 mm 0.2008 inch

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British Threads - BA

BA (British Association) Thread Data

Angle of thread = 47. 5 degrees, rounded at top and bottom