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British Threads - BA


BA (British Association) Thread Data

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 NumbersDiameter of Screw
Over Thread
Threads per inchPitchDiameter
at Thread Bottom
Tapping Drill Size
16 BA0.0310 inch133.3 tpi0.0075 inch0.0220 inchNumber drill 73
0.6 mm
0.0236 inch
15 BA0.035 inch120.5 tpi0.0083 inch0.025 inchNumber drill 70
0.7 mm
0.0276 inch
14 BA0.039 inch109.9 tpi0.0091inch0.028 inch0.8 mm
0.0315 inch
13 BA0.047 inch102 tpi0.0098 inch0.0352 inchNumber drill 62
0.98 mm
0.0386 inch
12 BA0.051 inch90.9 tpi0.011 inch0.0378 inchNumber drill 60
1.05 mm
0.0413 inch
11 BA0.059 inch81.9 tpi0.0122 inch0.0443 inchNumber drill 56
1.2 mm
0.0472 inch
10 BA0.067 inch72.6 tpi0.0138 inch0.0504 inchNumber drill 54
1.4 mm
0.0551 inch
9 BA0.075 inch65.1 tpi0.0154 inch0.0565 inch1.55 mm
0.061 inch
8 BA0.086 inch59.1 tpi0.0169 inch0.0657 inchNumber drill 50
1.8 mm
0.0709 inch
7 BA0.0982 inch52.9 tpi0.0189 inch0.0753 inchNumber drill 46
2.05 mm
0.0807 inch
6 BA0.11 inch47.9 tpi0.0209 inch0.0849 inch2.3 mm
0.0906 inch
5 BA0.126 inch43 tpi0.0232 inch0.0981 inchNumber drill 37
2.65 mm
0.1043 inch
4 BA0.1420 inch38.5 tpi0.0260 inch0.1108 inchNumber drill 31
3 mm
0.1181 inch
3 BA0.1610 inch34.8 tpi0.0287 inch0.1266 inchNumber drill 29
3.4 mm
0.1339 inch
2 BA0.1850 inch31.4 tpi0.0319 inch0.1467 inchNumber drill 22
4 mm
0.1575 inch
1 BA0.2090 inch28.2 tpi0.0354 inch0.1665 inchNumber drill 16
4.5 mm
0.1772 inch
0 BA0.2360 inch25.4 tpi0.0394 inch0.1887 inchNumber drill 9
5.1 mm
0.2008 inch