An assortment of socket-head cap screws from Hillman. Socket-head cap screws carry a recessed internal socket drive to add strength. They are commonly used on machine parts, manufacturing equipment, and other mechanical applications. Variants in this assortment feature a coarse 5/16"-18 thread size. Full assortments of #3218 contain eight (8) refills comprised of Hillman item numbers #3219 - #3226.