A full-bodied bolt with hex head, spaced threads and a gimlet point. Stainless: Has superior corrosion resistance to galvanized steel bolts. Length of a lag screw is measured from the underhead bearing surface to the extreme end of the screw. Lag bolts feature a hex head and are designed to be driven with a wrench or hex driver. Lag screws are also commonly called lag bolts because they are significantly larger than most other screws. Essentially, it is just a very large wood screw with the threads extending to the top. They feature the sharp cutting threads of a wood screw or sheet metal screw and have a shoulder below the head (in longer lengths), however everything is scaled up in size for use in larger applications. These screws typically need a pre-drilled whole and are not recommended as self-tapping.