5/16" Mechanical Zinc Finish Medium Split Lock Washer / HOLE DIAMETER: 0.318 INCHES MINIMUM AND 0.328 INCHES MAXIMUM OUTSIDE DIAMETER: 0.586 INCHES MAXIMUM MATERIAL THICKNESS: 0.078 INCHES MINIMUM AND 0.088 INCHES MAXIMUM HARDNESS RATING: 45.0 ROCKWELL C MINIMUM AND 53.0 ROCKWELL C MAXIMUM SPECIAL FEATURES: ZINC IS THE PREFERRED SURFACE TREATMENT. MIL SPEC STATES FINISH AS CADMIUM OR ZINC. MATERIAL: STEEL COMP 1060 OVERALL OR STEEL COMP 1561 OR STEEL COMP 1064 OR STEEL COMP 1065 OR STEEL COMP 1566 OR STEEL COMP 1069 OR STEEL COMP 1070 OR STEEL COMP 1572 OR STEEL COMP 1074 OR STEEL COMP 1075 OR STEEL COMP 1080 MATERIAL DOCUMENT AND CLASSIFICATION: 66 FED STD ALL MATERIAL RESPONSES OVERALL SURFACE TREATMENT: ZINC OVERALL SURFACE TREATMENT DOCUMENT AND CLASSIFICATION: QQ-P-416,TY 2,CL 3 FED SPEC SINGLE TREATMENT RESPONSE OVERALL STYLE DESIGNATOR: 108 SPLIT HELICAL-RIGHT HAND NONDEFINITIVE SPEC/STD DATA: 2 STYLE AND A CLASS