SERRATED FLANGE NUTS 1/4-20, 5/16-18, 3/8-16, Coarse ZINC PLATED STEEL HEX Rohs3, NC HARD/SERR. Serrated for extra grip. Nut with a wide flange. Picture shown above is example and each length have different amount of threads are on the shaft thread are decreases as length increases.
eBay