Hexagon nuts ISO 4032 5-2 AD W7 stamped M39

The wrench sizes for hexagon bolts and nuts are specified in ISO 272: 1982-01. The nominal wrench size on the workpiece (e.g. bolt, nut) is the same as that on the tool. However, there is some play between the two to allow the tool to be fitted without difficulty. The relationship between the nominal diameter d of the thread and the wrench size s is standardised in EN ISO 4014 and EN ISO 4017 for hexagon bolts, and in EN ISO 4032 for hexagon nuts (dimensions in mm).

For nuts, only a single digit is used, which indicates the maximum strength class of the bolt with which the nut can be used (a strength class 8 nut can be used with a 5.6 or 8.8 bolt, but not with a 10.9 bolt). If nuts with a strength class that is too low are used, there is a risk of the thread being stripped. The addition of ‘-2’ to the strength class designation excludes Thomas steel (a steel production process).

Steel is an iron-carbon alloy with a maximum carbon content of 2% by mass. The steels achieve their intended physical and mechanical properties through various hardening, annealing and tempering processes. It is also possible to harden the edge zone by surface hardening, case hardening or nitriding. Well-known steel grades are structural steels, free-machining steels, case-hardening steels, nitriding steels, tempering steels, tool steels and stainless steels.

The term plain indicates a completely untreated, merely oiled, smooth surface for all DIN and ISO screws such as wood screws, hexagon head screws, threaded rods, nuts and many types of washers.