M20 Form C Flat Washer BS4320C CR3 bzp

This is a plain flat washers form C which denotes the outside diameter of the flat washers The washers are BS4320 standard which are used  to spead the load when tightening fasteners so the nuts and bolts have a good contact surface to assist in keeping the nut or bolt secure.  Washers also take the pressure from just a small space by spreading the load from the component side to prevent wear in the connection of the fastening.

These are bright zinc plated fasteners which is a preventative measure from corrosion.  Avoid breaking into the bright zinc plating usually abbreviated as bzp as this will cause faster corrosion if not covered with a corrosive protected material as in time this will weaken the fastener.  Also bear in mind using different materials can cause fusing through corrosion especially in damp conditions when mixed with aluminium or stainless steel materials.

More information is available from Central Fasteners regarding fastener details, but its always worth checking with a manufacturer's specifications or drawings if an alternative size, tensile strength or types of washers are used.