A nyloc nut, also referred to as a nylon-insert lock nut, polymer-insert lock nut, or elastic stop nut, is a kind of locknut with a nylon collar that increases friction on the screw thread.
The nylon collar insert is placed at the end of the nut, with an internal width (ID) to some extent smaller than the main width of the screw. The screw thread does not cut into the nylon insert, however, the insert deforms elastically over the threads. The insert locks the nut against the screw as a result of friction, caused by the radial compression force resulting from the deformation of the nylon. Nyloc nuts retain their locking ability up to 250 °F (121 °C).