Hexagon Nuts and Plain Washers

The nut and the washer complete a bolted joint and are specified with the same care as the bolt. ISO 4032 covers hexagon regular nuts, style 1, in steel and stainless steel with metric coarse pitch thread from M5 to M39 in product grades A and B. Plain washers follow ISO 7089 for the normal series in product grade A, and ISO 7091 for the same series in the coarser product grade C; structural work to American practice uses ASTM F436 hardened washers.

The nut must be matched to the bolt in strength, not only in thread size. Carbon steel nuts carry property classes to ISO 898-2 and stainless nuts grades and classes to ISO 3506-2; the nuts in the product photograph are marked A2-70, that is the 304 stainless family at the 700 MPa class. A nut of a lower class than the bolt will strip before the bolt reaches its yield, which defeats the point of specifying a high class bolt.

The washer spreads the clamping force over a wider bearing area, protects the surface finish as the nut turns, and lets the nut be tightened without galling the joint face. In stainless assemblies the pairing matters for another reason: stainless threads gall when run dry under load, so a lubricant or anti-seize compound is normal practice on A2 and A4 fasteners.

When ordering, state the thread size, whether nuts and washers are wanted in stainless (A2 or A4) or in carbon steel with a stated property class, and the finish. Note that hot-dip galvanised bolts need galvanised, over-tapped nuts - ISO 898-1 refers hot-dip galvanised fasteners to ISO 10684 - and that a plain nut will not run onto a galvanised thread.