British Association or BA screw threads are a largely obsolete set of small screw threads, the largest being 0BA at 6mm diameter. A screw thread is a system used to turn rotational movement into linear movement with a geometric form similar to an inclined plane wrapped around a cylinder. ...
They are unusual in that they were originally defined in terms of the imperial units then in use in the United Kingdom, but were deliberately set to be exact multiples of the metric system units, rounded to the nearest thousanth of an inch. This anticipated metrication by about a century. This article is about post-1824 Imperial units, please see also English unit, U.S. customary unit or Avoirdupois. ... The International System of Units (symbol: SI) (for the French phrase Syst me International dUnit s) is the most widely used system of units. ... Thou may refer to: thou, the second person singular pronoun of the English language thou or mil, a unit sometimes used in engineering equivalent to one-thousandth of an international inch, and thus defined to be 25. ... Speedometer gauges on a car showing the speed of the vehicle in miles and kilometres per hour. ...
BA threads are still used in some precision instruments, such as optics and moving-coil meters.
External links
Bolt Science writeup.
Early history of non-metric bolts including BA and Whitworth.
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