This zinc plated hex nut has an enlarged circular base flaring out from the bottom of the nut. The bearing surface of the flange has serrations, which helps resist loosening when wrenched into place. This nut requires a greater amount of torque to loosen than to tighten, and it will span oversized or poorly aligned holes. Material: carbon steel with a maximum Carbon content of 0.47%, Phosphorus content of 0.12%, and Sulfur content of .23%. FNS516C2Z