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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 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
The size is imprinted on the spanners in millimeter values or inch sizes with intermediate sizes in fractions (older British and current US spanners). The two systems are in general not compatible, which can result in rounding of nuts and bolts (i.e. using a 13 mm (0.51 in) spanner in place of a 1 ⁄ 2 inch (12.70 mm)).
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 ...
Square head cap screws up to and including 3 ⁄ 4 inch (19.05 mm) have a head 1 ⁄ 8 inch (3.175 mm) larger than the shank; screws larger than 3 ⁄ 4 inch (19.05 mm) have a head 1 ⁄ 4 inch (6.35 mm) larger than the shank. [9] In 1919, Dyke defined them as screws that are threaded all the way to the head. [10] socket screw
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).
Machine screws with different thread diameters Hex key set with metric sizes from 2 mm to 10 mm according to sizes for use with ISO 262 screw sizes (except 3.5 mm for M6 internal set screw) For each size bolt or screw and type of head, there is a corresponding size driver prescribed by various ISO standards, including:
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.