Nylon insert lock nuts are self-locking nuts, six-sided nuts with nylon-filled collars at their back end. These collars adhere to the thread of the fastener bolt or screw they are being used with. When a bolt or screw reaches the collar, the threads and nylon form a tight, frictional fit, restricting movement of the fastener.

Nylon-insert locknuts are excellent for use where a long-term lock is necessary, or to prevent loosening due to vibration.