3/8-16 Solid Brass Hex Locknut Nylon Insert Elastic Stop Lock Nuts Qty 500
It is a nut with a nylon collar insert that resists turning. The insert deforms elastically over the threads of the screw, but threads are not cut into the nylon. This causes the nylon lock nut to grip better against the threads and reduces the chances of the nut loosening due to the vibration of the screw.