British Association or BA screw threads are a largely obsolete set of small screw threads, the largest being 0BA at 6 mm diameter. They were, and to some extent still are, used for miniature instruments and modelling.

They are unusual in that they were probably the most "scientific" design of screw, with a basis in metric size (the 1 mm pitch and 6 mm diameter of 0BA) and with a mathematical relationship between the 'number' (e.g. 2BA with K=2) and the corresponding pitch, major diameter, and then spanner size. Though originally defined in terms of the imperial units then in use in the United Kingdom, this was deliberately set to be exact multiples of the metric system units, rounded to the nearest thousandth of an inch. This anticipated worldwide metrication by about a century. The design was first formulated in 1884, following a study of Swiss practice and was standardised in 1903.

The British Standards Institution recommends the use of BA sizes in place of the smaller BSW and BSF screws (those below 1/4").[1]:4 Generally, the 0BA size was dropped in place of 7/32" BSF in assemblies that included larger fasteners, however, in smaller equipment that was primarily electronic/electrical the 0BA size would typically be used in place of the BSF or BSW screw where it was the largest size required.

BA threads are still used in some precision instruments, such as optics and moving-coil meters, relays etc. A 2BA thread is used to connect the head of a dart to its shaft — as such, it is one of the few common uses of this thread in North America.

2BA threads are commonly found in the old imperial BESA conduit boxes in the UK which are still in use (but not installed) today.

The pitch of 0BA is 1 mm and the pitch of each higher numbered thread is obtained by multiplying the pitch of the lower number by 0.9 so K-BA has a pitch of $\scriptstyle p=0.9^K$[1]:12. The major diameter is given by $\scriptstyle 6p^{1.2}$[1]:12 and the hex head size (across the flats) is 1.75 times the major diameter.

BA sizes are specified by the following British Standards:

• BS 93:1951 — Specification for British Association (B.A.) screw threads with tolerances for sizes 0 B.A. to 16 B.A.
• BS 93:2008 — British Association (B.A.) screw threads. Requirements

The shortening at the crest and root is given by 0.26817p with a radius of 0.18083p at both crest and root.[1]:2 The angle of the thread is 47.5 degrees[1]:2 which is different from that used by Whitworth (55 degrees) and ISO Metric (60 degrees) so BA fasteners are not properly interchangeable with Whitworth or metric ones even when the pitch and diameter are similar enough that they can be screwed together (e.g., although 0BA appears similar to M6x1mm, the male and female surfaces do not mate properly for bearing a load). Standard BA sizes are defined between 0BA and 22BA. The very small sizes are not used very often and other standards tend to be used (e.g., NHS).

BA Outer
Diameter[1]:7, 16
per inch
per mm
Tap drill
0 0.2362 in / 6.0 mm 25.38 1 5.1 mm / 7 gauge
1 0.2087 in / 5.3 mm 28.25 1.112 4.5 mm / 16 gauge
2 0.1850 in / 4.7 mm 31.35 1.234 4.0 mm / 21 gauge
3 0.1614 in / 4.1 mm 34.84 1.372 3.4 mm
4 0.1417 in / 3.6 mm 38.46 1.514 3.0 mm
5 0.1260 in / 3.2 mm 43.10 1.697 2.65 mm / 37 gauge
6 0.1102 in / 2.8 mm 47.85 1.884 2.3 mm
7 0.0984 in / 2.5 mm 52.91 2.083 2.05 mm / 46 gauge
8 0.0866 in / 2.2 mm 59.17 2.330 1.8 mm
9 0.0748 in / 1.9 mm 64.94 2.557 1.55 mm
10 0.0669 in / 1.7 mm 72.46 2.853 1.4 mm / 54 gauge
11 0.0591 in / 1.5 mm 81.97 3.227 1.2 mm
12 0.0512 in / 1.3 mm 90.91 3.579 1.05 mm

## Notes

1. British Standards Institution. British Association (B.A.) screw threads – requirements. ISBN 978-0-580-60893-3. BS 93:2008.