10x 2BA 1/2" Round Head, Slotted, Steel Machine Screws, Bolt & Nut Sets
Brand New

1/2" long, 0.18" thread width (diameter), 1/2" long, 13mm length by 4.6mm width thread (diameter).

British Association (BA) screw threads, named after the British Association for Advancement of Science, were devised in 1884 and standardised in 1903. Screws were described as "2BA", "4BA" etc.. 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 start end of the 20th century. 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. BA threads are specified by British Standard BS 93:1951. 

BA threads are still common in some niche applications. 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. 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. 5BA is also commonly used as it can be threaded onto 1/8 rod.