Patent ID: 7221079

Claim:
A spark plug comprising: a tubular metal shell having a threaded portion on an outer periphery thereof, the threaded portion having an outer diameter in a range of 12 to 14 mm; a cylindrical insulator fixed in said metal shell, said insulator having a bore, which is formed through said insulator in an axial direction of said insulator, and an end that protrudes from an end of said metal shell; a center electrode secured in the bore of said insulator, said center electrode having a first diameter portion, a second diameter portion, and an intermediate portion provided between the first and second diameter portions, the first diameter portion being positioned outside the bore of said insulator and having a free end, the second diameter portion having a diameter greater than that of the first diameter portion, the intermediate portion having a first interface with the first diameter portion and a second interface with the second diameter portion and tapering from the second interface to the first interface, the second interface of the intermediate portion with the second diameter portion being positioned outside the bore of said insulator; a first ground electrode having a base end joined to the end of said metal shell and a tip portion that faces the end of the first diameter portion of said center electrode through a first spark gap in the axial direction of said insulator; and a second ground electrode having a base end joined to the end of said metal shell and a tip portion that faces the second interface of the intermediate portion of said center electrode with the second diameter portion of the same through a second spark gap in a radial direction of said insulator, the tip portion of said second ground electrode also facing in the radial direction of said insulator a tip portion of said insulator which includes the end of said insulator; wherein the following dimensional relationships are defined: 0 <C <( A−B ); and 1.2 mm<( A−B ); where A is a minimum distance between an inner surface of the tip portion of said insulator and the tip portion of said second ground electrode in the radial direction of said insulator, B is a minimum distance between an outer surface of the tip portion of said insulator and the tip portion of said second ground electrode in the radial direction of said insulator, and C is a distance between the end of said insulator and the second interface of the intermediate portion of said center electrode with the second diameter portion of the same in the axial direction of said insulator.