These screws are of 410 untarnished steel, gear mechanism hardness -LRB- 40 Rockwell C -RRB-, resists chemical change in balmy environments, practical to steel-work. value hex washer head screw, can be easily with 1/4 `` hex driver. exact screw, the thread can decrease the resistance when screwing in, prevent cracking. There is no need to drill a hole first of all, musical notation tail slow to drill. : Screw Diameter : # 8 -LRB- 4.2 mm -RRB- ; Screw Length : 3/4 inches -LRB- 19 mm -RRB- ; Head diameter : 6.25 mm ; Head Thickness : 3.1 mm. head screw, play drilling and tapping at one time. Use the drill point for drilling first of all, then the screw for tapping and connection, which can save time and enhance efficiency. Package : 50pcs # 8x3/4 Inch Self-Drilling Hex Washer Head Tapping Screws .