Patent ID: 7703428

Claim:
A spark plug comprising: a central electrode, an insulator having an axial hole extending in the direction of an axial line and holding the central electrode at the leading end side of the axial hole, a metal housing including: a cylindrical hole extending in the direction of the axial line and holding at least part of the insulator in the cylindrical hole; a mounting part having screw threads to be screwed into a mounting hole of an internal combustion engine on an outer peripheral surface of the mounting part, the screw threads having a nominal diameter of 12 mm or less; a cylindrical part provided on a leading end side of the mounting part, the cylindrical part having a diameter smaller than a diameter of the mounting part and having a leading end surface which is to be disposed inside the mounting hole; a first tapered part connecting an outer peripheral surface between the cylindrical part and the mounting part in the vicinity of the leading end of the mounting part in a tapered shape; a flange part provided on the base end side of the mounting part, the flange part having a diameter larger than a diameter of the mounting part; and a second tapered part connecting an outer peripheral surface between the mounting part and the flange part in a tapered shape, wherein the ratio B/A is 0.2 or greater; B is 2 mm or greater, and C−B is 1.5 mm or greater when A is an axial length from a leading end of the second tapered part to the leading end surface of the cylindrical part, B is an axial length from the leading end of the first tapered part to the leading end surface of the cylindrical part, and C is the maximum axial length from the leading end of the first tapered part to an opening end on the combustion chamber side of the mounting hole when the metal housing is screwed into the mounting hole of the internal combustion engine.