Hexagon Socket cap Bolt: the spiral of head outside is round, the middle is concave into the hexagon.
Hex Socket Screw DIN912/grade 12.9 Hex Socket Screws
Model Number: DIN912ASME B18.3BS 2470
M1.6--M36#4-1"1/8-1'
Length: 3MM--300MM
Strength Grade: 12.910.98.8
Material:Alloy SteelCarbon Steel Carbon SteelCarbon Steel