BSW Whitworth 5/16" Socket Cap Screw High Tensile 12.9 DIN 912 BSW, Whitworth 5/16", Allen/ Hexagon Socket, Cap Head, Screw/ Bolt, High Tensile (12.9), BS 2470. What is a socket cap screw/ bolt?. A screw is fully threaded and a bolt is partialy threaded.