1000pcs 9/16"-18 Hex Jam Nuts, Zinc Plated Steel Lock Nuts for Automotive, Machinery & DIY Projects - High Strength Fasteners for Secure Connections

A six-sided internally threaded fastener which is only 2/3 the thickness of a full form nut

Jam nuts are tightened against the work surface and a finished or heavy hex nut is tightened agianst the jam to keep it from loosening