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BA Thread

British Association fastener thread

Bolts, nuts, screws and fastenings with BA threads HISTORY

BA thread

BA......	TPI Threads per inch	Effective diameter	Major Diameter	PITCH	Pitch Diameter	Minor Diameter male thread	Thread Height female thread	Tap Drill
			d=D	P	d2=D2	d1	h
0BA	25.4	0.2126	0.2362	0.0394	0.2110	0.1890	0.0236	0.1969
1BA	28.2	0.1874	0.2087	0.0354	0.1860	0.1661	0.0213	0.1732
2BA	31.4	0.1659	0.1850	0.0319	0.1646	0.1469	0.0191	0.1535
3BA	34.8	0.1441	0.1614	0.0287	0.1430	0.1268	0.0173	0.1339
4BA	38.5	0.1262	0.1417	0.0260	0.1251	0.1106	0.0156	0.1161
5BA	43.1	0.1120	0.1260	0.0232	0.1111	0.0980	0.0140	0.1024
6BA	47.9	0.0976	0.1102	0.0209	0.0969	0.0850	0.0126	0.0906
7BA	52.9	0.0870	0.0984	0.0189	0.0863	0.0756	0.0114	0.0787
8BA	59.1	0.0764	0.0866	0.0169	0.0758	0.0661	0.0102	0.0709
9BA	65.1	0.0656	0.0748	0.0154	0.0650	0.0563	0.0093	0.0591
10BA	72.6	0.0587	0.0669	0.0138	0.0581	0.0504	0.0083	0.0531
11BA	81.9	0.0518	0.0591	0.0126	0.0512	0.0445	0.0073	0.0472
12BA	90.7	0.0445	0.0512	0.0110	0.0441	0.0378	0.0067	0.0394
13BA	101.6	0.0413	0.0472	0.0098	0.0409	0.0354	0.0059	0.0374
14BA	110.4	0.0339	0.0394	0.0091	0.0336	0.0283	0.0055	0.0295
15BA	121.0	0.0305	0.0354	0.0083	0.0302	0.0256	0.0049	0.0276
16BA	133.7	0.0266	0.0311	0.0075	0.0263	0.0220	0.0045	0.0236
17BA	149.4	0.0236	0.0276	0.0067	0.0233	0.0197	0.0039	0.0209
18BA	169.3	0.0209	0.0244	0.0059	0.0206	0.0173	0.0035	0.0185
19BA	181.4	0.0179	0.0213	0.0055	0.0177	0.0146	0.0033	0.0157
20BA	211.7	0.0161	0.0189	0.0051	0.0159	0.0134	0.0028	0.0142
21BA	230.9	0.0140	0.0165	0.0043	0.0138	0.0114	0.0026	0.0122
22BA	254.0	0.0122	0.0146	0.0039	0.0121	0.0098	0.0024	0.0106
23BA	282.0	0.0108	0.0130	0.0035	0.0107	0.0087	0.0022	0.0094
24BA	317.0	0.0094	0.0114	0.0031	0.0094	0.0075	0.0020	0.0083
25BA	363.0	0.0063	0.0098	0.0028	0.0081	0.0067	0.0016	0.0071

HISTORY

BA Threads

BA (British Association) screw threads are named after the British Association for Advancement of Science and were put together in 1884 and standardised and published in 1903.

Screws were described as 2BA, 4BA, 6BA and 8BA; the bigger numbers denote smaller screws. The odd and bigger numbers being rarely used, except in equipment made prior to the 1970s for telephone exchanges in the UK making extensive use of odd-numbered BA screws, in order to reduce theft.

BA threads are specified by British Standard BS 93:1951 sizes 0 B.A. to 16 B.A.

The sizes were actually defined in metric terms, an 0BA thread having a 6 mm diameter and 1 mm pitch. Other threads in the BA series are related to 0BA in a geometric series with the common factors 0.9 and 1.2. For example, a 4BA thread has pitch 0.65mm and diameter 3.62mm. Although 0BA has the same diameter and pitch as an M6 coarse, the threads have different forms and are not compatible.

British Association

BA threads are still common in some applications, certain types of fine machinery, such as moving-coil meters and clocks, tend to have BA threads wherever they are manufactured. BA sizes were also used extensively in aircraft, especially those manufactured in the United Kingdom. BA sizing is still used in railway signalling, mainly for the termination of electrical equipment and cabling.

BA threads are extensively used in Model Engineering where the smaller hex head sizes make scale fastenings easier to represent. As a result many UK Model Engineering suppliers still carry stocks of BA fasteners up to typically 8BA and 10BA. Nuts in size 5BA are extensively used as they can be threaded onto 1/8" rod.

BA Thread

British Association fastener thread

BA Threads

British Association

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