Parallel pins m6 ISO 8734-steel hardened, Type A, 25x18 mm

The tolerance grade is indicated by numbers ranging from 1 to 18. As the number increases, the size of the tolerance zone increases. The size of the tolerance depends on the nominal dimension. The tolerance system for internal and external dimensions is standardised in accordance with DIN EN ISO 286. The tolerance zones for external dimensions are indicated by lower-case letters followed by numbers.

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.