5/16"-18 Hex Tap Bolts in Silicon Bronze. Silicon Bronze offers superior corrosion resistance making these tap bolts suitable for salt water, marine or harsh environment applications. Hex tap bolts are also known as tap screws and are used in applications where the shank is fully threaded to the underside of the hex head. Hex tap bolts have less strength than hex cap screw due to the full thread. Tap bolts are also like over-sized machine screws with larger hex heads for strength.