ISO International Standard

ISO 691:2005 specifies tolerances on wrench and socket openings for screws and nuts (or similar parts) having metric width across flats dimensions. It gives, for each dimension, the tolerances for general use, defined by its two deviations, minimum and maximum, with respect to the nominal value. These deviations have been determined in relation to the width across flats tolerances for screws and nuts specified in ISO 4759-1 and with regard to the manufacture of tightening tools.