Patent ID: 8079243

Claim:
A plug for use in expanding an inside diameter of an end portion of a metal pipe or tube, said plug having a circular cross section and including a taper portion and a parallel portion connected to a tail end of said taper portion, wherein a diameter of said taper portion gradually increases from a head end of said taper portion to the tail end of said taper portion where the diameter at the tail end of the taper portion is D 1 , an axial distance LR from a point of said taper portion where the diameter is D 2 , wherein D 2 =D 1 ×0.99, to the tail end where the diameter is D 1 satisfies Expression (1): 22≦ LR /( D 1− D 2)/2)≦115 (1) a taper angle on a surface of the taper portion where the diameter of the taper portion is D 2 is larger than or equal to a taper angle on a tail surface of said taper portion following a point where the diameter is D 2 , and a diameter of said parallel portion is D 1 .