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A few sizes are close enough to interchange for most purposes, such as 19 mm (close to 3 ⁄ 4 inch (19.05 mm)), 8 mm (close to 5 ⁄ 16 inch (7.94 mm)) and 4 mm (close to 5 ⁄ 32 inch (3.97 mm)). In reality, a wrench with a width across the flats of exactly 15 mm would fit too tightly to use on a bolt with a width across the flats of 15 mm.
Cap head screws of various sizes Button head screws. Hex keys are best known as "Allen keys" or "Allen wrenches" in English-speaking countries like the UK, Australia, Canada and the U.S. and in Spanish-speaking countries such as Spain and Mexico (The "Allen" name is a registered trademark, originated by the Allen Manufacturing Company of ...
Confusion can arise because each Whitworth hexagon was originally one size larger than that of the corresponding BSF fastener. This leads to instances where for example, a spanner marked 7 ⁄ 16 BSF is the same size as one marked 3 ⁄ 8 W. In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same.
The major diameter is given by 6p 1.2, [4]: 12 rounded to two significant figures in mm and the hex head size (across the flats) is 1.75 times the major diameter. BA sizes are specified by the following British Standards: BS 57:1951 — B.A. screws, bolts and nuts
A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by a dash or sometimes the multiplication sign, × (e.g. M8-1.25 or M8×1.25).
The major minus pitch technique also works for inch-based threads, but you must first calculate the pitch by converting the fraction of threads-per-inch (TPI) into a decimal. For example, a screw with a pitch of 1/20 in (20 threads per inch) has a pitch of 0.050 in and a 1 ⁄ 13 in pitch (13 threads per inch) has a pitch of 0.077 in.
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In 1913, Woolley and Meredith defined them like Anthony, but gave the following dimensions: hex head cap screws up to and including 7 ⁄ 16 inch (11.1125 mm) have a head that is 3 ⁄ 16 inch (4.7625 mm) larger than the shank diameter; screws greater than 1 ⁄ 2 inch (12.7 mm) in diameter have a head that is 1 ⁄ 4 inch (6.35 mm) larger than ...