Flat Washer Flat Washer Type Standard Dimensional Standards DIN 6916 Material Steel Material Grade/Class Grade 8 Finish Black Oxide For Screw Size M30 Inside Diameter 31 mm Outside Diameter 56 mm Maximum Thickness 5 mm System of Measurement Metric Minimum Thickness 5 mm Pack Size Small Pack Rockwell Hardness C29 Thickness Tolerance +/-0.600mm