2812 Series Zinc-Plated Steel Nylon-Insert Locknut (M4 x 0.7mm, 7mm Hex) - 25 Pack

These locknuts have a nylon insert that the threads of a mating screw must form threads in upon insertion.  The displaced nylon will continue to hold tension on the screw threads which act as a locking mechanism to prevent the screw from loosening under vibration.