... 6.8,8.8,9.8..... SOCKET HEAD SELF TAPPING SCREW,COUNTERSUNK HEAD PHILLIPS WOOD SCREW,GALVANIZED SELF TAPPING SCREW. ... for Machine threads.Head typesHexagon head,Mushroom head,Pan head,Countersunk head,Bugle head,Oval head,Hex flange head,Round head,Ball head,Button head...Socket typesPhillip type, ... 0.23% ,is higher hardness than other.2.Fasteners' Head is much thicker ,it is strong not break down . ...