Patent ID: 7183702

Claim:
A spark plug comprising: a tubular metal shell having a first end and a second end opposed to the first end, said metal shell also having a threaded portion on an outer periphery thereof and an annular seat formed on an inner surface of said metal shell, the threaded portion having an outer diameter of 10 mm or less; a hollow insulator having a length with a first length portion, a second length portion, and a shoulder provided between the first and second length portions, the shoulder having an outer surface that tapers and continues to an outer surface of the first length portion, said insulator being fixed in said metal shell such that the shoulder of said insulator and the annular seat of said metal shell are in sealing engagement through a gasket, the first length portion of said insulator having an end protruding from the first end of said metal shell; a center electrode secured in said insulator, said center electrode having an end protruding from the end of the first length portion of said insulator; and a ground electrode joined to the first end of said metal shell, said ground electrode having a tip portion that faces the end of said center electrode through a spark gap, wherein the first length portion of said insulator tapers toward the end thereof to have a first outer diameter on a first reference plane and a second outer diameter on a second reference plane, the first reference plane being defined to extend perpendicular to a lengthwise direction of said insulator through an intersection between a first reference straight-line having a section on the outer surface of the shoulder and a second reference straight-line having a section on the outer surface of the first length portion of said insulator, the second reference straight-line being so defined that the first length portion of said insulator has a maximum outer diameter on the section of the second reference straight-line, the second reference plane being defined to extend parallel to the first reference plane through an inner edge of the first end of said metal shell, wherein the following dimensional relationships are defined: D−D 0≧1.0 mm; T0≧1.2 mm; G≦0.9 mm; and D 4 − D 0≧0.8 mm, where D is the first outer diameter of the first length portion of said insulator on the first reference plane, D 0 is the second outer diameter of the first length portion of said insulator on the second reference plane, T 0 is a distance between the inner surface of said metal shell and the outer surface of said insulator on the second reference plane, G is a space of the spark gap between the end of said center electrode and the tip portion of said ground electrode, and D 4 is an outer diameter of the first length portion of said insulator on a fourth reference plane that is defined to extend parallel to and spaced a distance of 1.5×T 0 from the second reference plane.