5/16" x 0.688" OD Thru-Hardened Yellow Zinc Finish Steel SAE General Purpose Flat Washer / CROSS-SECTIONAL SHAPE STYLE: 1 RECTANGULAR HOLE DIAMETER: 0.344 INCHES NOMINAL PERIPHERAL SHAPE STYLE: 1 ROUND CENTER HOLE SHAPE STYLE: 1 ROUND OVERALL DIAMETER: 0.688 INCHES NOMINAL END ITEM IDENTIFICATION: MRAP - UNIQUE NSN - USED ON 3 MRAP VARIANTS; MRAP - COUGAR ISS 1.0; MRAP - COUGAR ISS 1.5; MRAP - M-ATV SOCOM; MRAP - M-ATV UI; MRAP - COUGAR CAT1 A2; MRAP - COUGAR AMBULANCE WIDTH ACROSS FLATS: 0.065 INCHES NOMINAL HARDNESS RATING: 38.0 ROCKWELL C MINIMUM AND 45.0 ROCKWELL C MAXIMUM SPECIAL FEATURES: 5/16"SAE ZINC PLATED THRU HARD HIGH STRENGTH FLAT WASHER MATERIAL: STEEL OVERALL MATERIAL DOCUMENT AND CLASSIFICATION: ASTM F436 ASSN STD SINGLE MATERIAL RESPONSE OVERALL PROPRIETARY CHARACTERISTICS: NPAC SURFACE TREATMENT: ZINC OVERALL AND DICHROMATE SURFACE TREATMENT DOCUMENT AND CLASSIFICATION: ASTM B633, TYPE II, CLASS 3 ASSN STD SINGLE TREATMENT RESPONSE OVERALL