BA, BSA

British Association screw thread series

In 1884 the British Association for the Advancement of Science adopted a thread form and series, primarily for use in precision equipment. It was inspired by one used in the Swiss watch and clock industry, and was formerly sometimes called the Swiss Small Screw Thread System; but is now just referred to as the BA series. Like the Whitworth thread, it has a rounded root and crest, but the included angle is 47.5° and the radius was originally 2/11 (0.1818…) times the pitch, but at some later point was changed to 0.18083. The gage numbers and pitch are related, roughly, by pitch = 0.9 to the power of the gage number.

Despite the date of origin and the British sponsorship, the BA is a metric series. Not only is it based on the meter, but the thread frequency is specified in terms of round numbers in the pitch sequence, with threads per meter allowed to fall where it may.

The British Association thread played a role similar to that of the ASME series in the United States, that is, it filled in sizes below a quarter-inch. The sizes most used were the even-numbered ones between 0 and 10, inclusive. The very small sizes, from 17 up, were rarely used. The British Standards Institution (BSI) discouraged the use of #0 BA in favor of the ⁷⁄₃₂″ BSF.

The BSI recommended this series replace the BSW and BSF series for all screws smaller than ¼ inch, and deprecated the use of the odd-numbered gages. The ¼ inch BSF was retained and the 0 gage BA discontinued (the two were too easily confused).

In 1966 the BSI declared the BA screw thread obsolete; its place was taken by ISO screw threads.

Gage Number	Major diameter (mm)	Threads per inch (approx)	Pitch (mm)	Tap drill
25	0.25	363	0.070	—
24	0.29	317	0.080	—
23	0.33	282	0.090	—
22	0.37	254	0.098	—
21	0.42	231	0.11	—
20	0.48	212	0.12	80
19	0.54	181	0.14	78
18	0.62	169	0.15	76
17	0.70	149	0.17	75
16	0.79	134	0.1900	74
15	0.90	121	0.2100	72
14	1.0	110	0.2300	70
13	1.2	102	0.2500	65
12	1.3	90.7	0.2800	61
11	1.5	82.0	0.3100	56
10	1.7	72.6	0.3500	54
9	1.9	65.1	0.3900	53
8	2.2	59.1	0.4300	49
7	2.5	52.9	0.4800	46
6	2.8	47.9	0.5300	43
5	3.2	43.0	0.5900	37
4	3.6	38.5	0.6600	31
3	4.1	34.8	0.7300	29
2	4.7	31.4	0.8100	23
1	5.3	28.2	0.9000	17
0	6.0	25.4	1.0000	10

Randall C. Brooks.
Standard screw threads for scientific instruments. Part II: The British Association Screw Gauge.
History and Technology, volume 6, pages 45-59 (1988).

British Standard 93-1919 (formerly CL 7271). Screw Threads, British Association, with Tolerances for sizes No. 0 to 15 B.A.. An addendum was issued in August 1940.

British Standard 93:1951.