Metric nuts and bolts are commonly referenced using 'M' sizes, for example: M3, M8, M12. The number '8' refers to the nominal diameter of the bolt shaft in millimeters. The number 1.0 is the thread pitch of the bolt, which is the distance between threads, in millimeters.
Pros of Using A Serrated Flange Nut You can leave out washers when using this type of nut, since the flange extends the bearing surface and those serrations need direct contact with the mating surface.
Where can you use flange nuts?
A flange nut is used inside a specially shaped dished-out washer. The device is commonly used in the aerospace industry. If this nut were not used the object would have to be spot faced so as to provide a surface perpendicular to the fastener.