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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.
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
Even though #14 and #16 screws are still available, they are not as common as sizes #0 through #12. [citation needed] Sometimes "special" diameter and pitch combinations (UNS) are used, for example a 0.619 in (15.7 mm) major diameter with 20 threads per inch. UNS threads are rarely used for bolts, but rather on nuts, tapped holes, and threaded ODs.
The major minus pitch also works for inch-based threads, but you must first determine the pitch by looking at the number of treads per inch (TPI; for example, 1 ⁄ 20 = 0.050 and 1 ⁄ 13 ≈ 0.077), and your result will only land near a tap drill size (not directly on one). Example (inch coarse): For 7 ⁄ 16-14, 1 in ÷ 14 = 0.071 in; 0.437 ...
British Standard Fine (BSF) is a screw thread form, as a fine-pitch alternative to British Standard Whitworth (BSW) thread. It was used for steel bolts and nuts on and in much of Britain's machinery, including cars, prior to adoption of Unified, and later Metric, standards.
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.
For each size bolt or screw and type of head, there is a corresponding size driver prescribed by various ISO standards, including: Internal hex drive: ISO 2936:2014 "Assembly tools for screws and nuts—Hexagon socket screw keys" [5] External hex drive: ISO 4014, 4016, 4017, and 4018 [6]
Style 1 hexagon nuts with metric fine pitch thread; property classes 6 and 8: Withdrawn: DIN EN ISO 8673: ISO 8673: DIN 971-2: Style 2 hexagon nuts with metric fine pitch thread; property classes 10 and 12: Withdrawn: DIN EN ISO 8674: ISO 8674: DIN 972: M5 to M39 hexagon nuts; style 1; product grade C (modified version of ISO 4034) Withdrawn ...