its radial thickness is in the range of 1 83 1 77 mm and the reference outside diameter of the seal is 47 26 mm the nominal inside diameter of the seal is 1 789 inches and its nominal radial thickness is 0 0728 inches after a seal is formed as shown in fig3 5 to the dimensions mentioned above the seal is plastically deformed by inserting a mandrel 65 through the inside surface of the seal the mandrel 65 has an outer surface in the form of a right circular cylinder whose diameter exceeds the diameter of the seal as the seal is originally formed as shown in fig3 5 a purpose of the plastic deformation of the seal is to form a gap y between the ends of the seal when the seal is in a free condition as shown in fig6 the mandrel 65 remains inserted in the seal for a period of at least 30 minutes or more the gap y formed by this method preferably should not exceed 0 80 inch this action plastically deforms the seal particularly in the area 64 located about 180 degrees from the split and extending ten degrees in both angular directions from that location the size of the gap y produced by plastic deformation of the seal by the mandrel 65 when that gap is measured after the seal is removed from the mandrel and the seal remains in a free state for at least one hour can be determined by the following empirical regression equation y u2212.