A clamp head including an arched body having upper and lower faces, and opposing unequal length arms. The body further has a through aperture for a bolt or like threaded fastener extending between the upper and lower faces, the through aperture tapering outwardly through the body in the direction of the lower face. The arms comprise a reaction arm having at its distal end a curved reaction face and a clamping arm having at its distal end a curved clamping face. The curved reaction face has a first edge that is proximal to the reaction face and a second edge that is distal to the reaction face, wherein the radius of curvature of the reaction face increases in the direction of the second edge.