[M4(4.5mm) outer diameter 20mm thickness 1mm]
[M4(4.5mm) outer diameter 16mm thickness 1mm]
The washer has a larger outer diameter than the standard size flat washer, so it is expected to reduce the phenomenon of the nut or bolt head into the clamping material (seated recess) and stabilize the tightening surface.
Besides metal, it is also suitable for fastening FRP and wood etc.