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British Association or BA screw threads were designed for use in modelling and instrumentation
Often used in British motorcycles where threads less than 1/4 inch diameter are required, such as electricals, badge screws, carburettors etc.
Counterintuitively, as the BA number increases, the diameter decreases, and thread pitch gets finer... as below
| (BA No.) | Diameter | Threads per inch (TPI) |
| 0BA | 0.2362 inch (6.00mm) | 25.40 tpi |
| 1BA | 0.2087 inch (5.30mm) | 28.22 tpi |
| 2BA | 0.1850 inch (4.70mm) | 31.36 tpi |
| 3BA | 0.1614 inch (4.10mm) | 34.79 tpi |
| 4BA | 0.1417 inch (3.60mm) | 38.48 tpi |
| 5BA | 0.1260 inch (3.20mm) | 43.05 tpi |
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