Stainless Steel Nyloc Nut
A nut with a plastic insert which deforms on the thread increasing the friction and applying a compressive force which effectively prevents the nut from working loose. There is some debate as to whether these nuts should be re-used. Certainly after a few uses the effectiveness can be felt to be reduced. They are not suitable in temperatures above about 120°C when single coil or shakeproof washers are preferred.