80mm Lag Screw to suit timber & concrete Due to its large size, hex head and deep coarse thread, the Lag Screw was originally deemed to be a Lag Bolt. The contrast between this definition and the idea that a nut is a receptacle of a bolt is what eventually lead to the redefinition. Also known in the United Kingdom as a Coach Screw, the gauge of the shank combined with the course thread allow for heavy duty timber and concrete fastening applications.