The elliptical head refers to a head composed of half an elliptical shell and straight sides (cylindrical short sections). Because the radius of curvature of the elliptic curve changes continuously, the elliptical shell produces only a thin film stress in its overall part after being compressed. Because of the common tangent at the joint of the elliptical shell and the straight side, the discontinuous stress is small. For a standard elliptical head with the ratio of long and short axis equal to 2, the edge stress at the junction of the elliptical shell and the straight side is small, so it can be ignored.