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British Association Threads

A British system of threads with 47.5° included angle with rounded roots and crest. Even numbers are the preferred sizes.

Title
B.A.	Tapping Drill		Clearance Drill
	inches	mm	inches	mm
0	0.189	4.8	0.242	6.147
1	0.166	4.216	0.213	5.410
2	0.147	3.734	0.191	4.851
3	0.1285	3.264	0.166	4.216
4	0.111	2.819	0.147	3.734
5	0.098	2.489	0.136	3.454
6	0.086	2.184	0.116	2.946
7	0.076	1.930	0.104	2.462
8	0.067	1.702	0.0935	2.375
9	0.0595	1.511	0.081	2.057
10	0.0465	1.181	0.073	1.854
11	0.042	1.067	0.067	1.702
12	0.037	0.940	0.055	1.397
13	0.035	0.889	0.055	1.397
14	0.028	0.711	0.0469	1.191
15	0.025	0.635	0.040	1.016
16	0.0225	0.572
17	0.020	0.508
18	0.018	0.457
19	0.0145	0.368

Reference

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

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