Flat Washer Flat Washer Type Standard Material Stainless Steel Material Grade/Class 18-8 Finish Plain For Screw Size M20 Inside Diameter 21 mm Outside Diameter 35 mm Maximum Thickness 3.5 mm System of Measurement Metric Minimum Thickness 3.5 mm Inside Diameter Tolerance +/-0.010 in Material Quality Standards ASTM A582 Outside Diameter Tolerance +/-0.010 in Pack Size Small Pack Resistance Properties Chemical Resistant Corrosion Resistant Rockwell Hardness B83