Check the listing for details. 5/16"-18 x 2" 18-8 Stainless Socket Cap Screws (6 pcs.). Condition: New. Listed at 16.38 USD. Diameter: 5/16" Length: 2" Material: Stainless Steel Head Style: Socket (1/4" Socket) Thread Size: 18 (Coarse) Hardness: 18-8 Type: Socket Cap Screw Box of 6. A socket cap screw is a type of cap screw with a hexagonal recessed drive. A hex key is required to tighten or loosen a socket screw. Socket screws are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket.