The plastic region bolt is used in such areas as the cylinder head or bearing cap assembly to provide a steady bolt tension. Normally bolts are tightened to the elastic region. In this condition, the bolt is tightened to the specified tightening torque. In the elastic region, the bolt tightening torque and bolt tension becomes greater proportionally. For tightening a bolt in the elastic region, some tolerance will be created through the bolt thread, flange or washer if the bolt tension is controlled at the tightening torque. In the plastic region, there is almost no change in bolt tension from tightening torque. The plastic region tightening method uses the material properties so that the uneven bolt tension from fluctuations in tightening torque is reduced. Bolt tension is stabilized as bolt tension itself becomes greater.