Jam nylon insert Locknuts keeps fasteners in place through the nylon insert in the center of the locknut.  The nylon piece grips tightly around the threads of a fastener and prevents it from backing out or loosening.  The jam nylon insert locknut is half the height of a standard nylon insert locknut.  Add these locknuts where low clearance applications apply or behind a standard hex nut to prevent loosening.  These are reusable but the holding power decreases with each use.