Coach bolts, frequently called coach screws, are self tapping fasteners with the head of a hexagon bolt. Coach bolts are commonly used in fastening wood to metal and wood to wood. The unthreaded section of a coach bolt adds strength to its construction, and makes it more resistant to shear forces when compared to fully thread variants such as self tapping hexagon bolts.
While specialised drives such as the pozi drive are specially designed to resist cam-out, the sturdy hexagon head of a coach bolt is even more resistant to slippage, and is typically more suitable for heavy-duty applications.
Due to their suitability for more heavy-duty applications and their compatibility with larger tooling, coach bolts are generally available in larger sizes than other self tapping screws.