The bolt shank diameter and the number of bolts are determined based on the torque transmitted by the coupling joint. In service, the coupling bolts may gall at the contact point between the inner faces of the nuts and the outer face of the coupling. Galling could cause uneven bolt diameters along the body of the bolt (such as variable diameters at points 1 and 2 in Fig. 6.17). This could lead to loosen couplings and eventual misalignment. So, coupling bolts are made of high-strength material to match with the coupling flange material