Patent ID: 7282845

Claim:
A spark plug for an internal combustion engine comprising: a tubular metal shell having an axis, said metal shell also having a first end portion to be disposed in a combustion chamber of an internal combustion engine and a second end portion opposed to the first end portion; a cylindrical insulator fixed in said metal shell, said insulator having a first end exposed outside the first end portion of said metal shell and a second end opposite to the first end, said insulator also having a plurality of bores formed therein which extend from the first end to the second end of said insulator; a plurality of center electrodes each of which is retained within one of the bores in said insulator, said center electrodes each having an axis; a plurality of thick-walled portions formed in the first end portion of said metal shell, a plurality of ground electrodes each of which is joined to one of said plurality of thick-walled portions; and a plurality of thin-walled portions formed in the first end portion of said metal shell, each of said thin-walled portions having an inner surface which is outwardly recessed in an angular range that includes an intersection of the inner surface with a reference line, the reference line being defined to extend from the axis of said metal shell through the axis of one of said plurality of center electrodes on a reference plane that is defined to extend perpendicular to the axis of said metal shell through an inner edge of the first end portion of said metal shell.