5/16"-18 18-8 Stainless Steel Coarse Thread Serrated Lock Nuts (8 pcs.)

Diameter: 5/16"
Diameter 2: 9/16"
Thickness: 9/32"
Material: Stainless Steel
Driver Type: 1/2" Wrench
Thread Size: 18 (Coarse)
Hardness: 18-8
Style: Serrated
Type: Lock Nut
Diameter 3: 5/8"

Box of 8.

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