The "M" denotes a standard-series profile (typically with a 14 mm internal bore) used to distribute clamping force.
NF typically refers to National Fastener specifications, The "Black" designation typically refers to a chemically blackened or zinc flake protective surface finish, providing a matte look and mild corrosion resistance.
The zinc finish acts as a sacrificial barrier to protect the underlying steel from rust and corrosion.
It is primarily suited for indoor, dry, or mildly corrosive environments, as constant exposure to moisture will wear the zinc layer down
These washers are used under bolt heads or nuts to distribute clamping force over a larger area, preventing damage to the mating surfaces and halting bolt pull-through.
Because of their strict Grade A tolerances, they are widely used in critical European automotive and industrial assemblies
