5/16"-18 Black Phosphate Grade 2 Steel Coarse Thread Lock Nuts

Diameter: 5/16"
Diameter 2: 9/16"
Thickness: 17/64"
Material: Steel
Finish: Black Phosphate
Driver Type: 1/2" Wrench
Thread Size: 18 (Coarse)
Hardness: Grade 2
Style: Top Lock|Type C
Type: Lock Nut

Box of 509.

A lock nut is a nut that resists loosening under vibrations and torque.