Abstract: A spark plug includes a metallic shell having a cylindrical trunk portion with a male screw formed on its outer circumferential surface and a seat portion located adjacent to the rear end of the trunk portion and projecting radially outward. The trunk portion has a first region and a second region. The first region extends, in the direction of the axial line, at least over a range extending from the rear end of the trunk portion to a first screw thread portion of the male screw as counted from the rear end of the male screw. The second region is located adjacent to the first region in the circumferential direction. The first region has a Vickers hardness higher than that of the second region.

Abstract: A spark plug includes a metallic shell having a cylindrical trunk portion with a male screw formed on its outer circumferential surface and a seat portion located adjacent to the rear end of the trunk portion and projecting radially outward. The trunk portion has a first region and a second region. The first region extends, in the direction of the axial line, at least over a range extending from the rear end of the trunk portion to a first screw thread portion of the male screw as counted from the rear end of the male screw. The second region is located adjacent to the first region in the circumferential direction. The first region has a Vickers hardness higher than that of the second region.

Abstract: An ignition plug includes an insulator having an outer-diameter reduction surface and a metal shell. The metal shell includes a rear end portion, a curved deformation portion, and a stepped portion. In a section including an axial line, the average hardness of a surface layer portion, which extends along a specific surface, of a rear end surface of the stepped portion is higher than the average hardness of the rear end portion and the average hardness of the deformation portion.

Abstract: An ignition plug includes an insulator having an outer-diameter reduction surface and a metal shell. The metal shell includes a rear end portion, a curved deformation portion, and a stepped portion. In a section including an axial line, the average hardness of a surface layer portion, which extends along a specific surface, of a rear end surface of the stepped portion is higher than the average hardness of the rear end portion and the average hardness of the deformation portion.