Hex bolts are bolts with a hexagonal head on one end, with no washer face on the bearing surface, and a threaded shaft on the other end; hex bolts are tightened with a wrench, they are used to connect metal parts. (The body tolerances of hex bolts are not as close as those of hex cap screws.)
Also known as the Hex Head Bolts, the Hex Head Machine Bolts, or Machine Bolts, Hex Bolts offer easier wrenching than square bolts, and is much more common in present-day applications. Sizes run from ¼-20 to 2-4. Machine bolt shafts may be partially threaded or fully threaded.
Basic thread length equals twice the bolt diameter (size) plus 1/4 inch for machine bolt lengths up to and including 6 inches, and twice the bolt diameter plus 1/2 inch for lengths over 6 inches.
The head of the Heavy Hex Bolt is wider and thicker than that of the hex bolt; the Heavy Hex Bolt goes with a heavy hex nut.
The Square Head Bolt, also known as the Square Head Machine Bolt, offers less danger of wrench slippage in hand wrenching, as well as a greater bearing area. Finally, a square head is more secure than other machine bolt/hex bolt styles when fixed into a cored recess.