Patent ID: 7170219

Claim:
A spark plug comprising: a tubular metal shell having a threaded portion on an outer periphery thereof, the threaded portion having a size less than or equal to M 14 ; an insulator fixed in said metal shell, said insulator having a bore formed therethrough and an end that protrudes from an end of said metal shell; a cylindrical center electrode secured in the bore of said insulator, said center electrode having an axis and an end that protrudes from the end 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 said center electrode through a first spark gap in an axial direction of said center electrode; and a second ground electrode having a base end joined to the end of said metal shell and a tip portion that faces an outer side surface of said center electrode through a second spark gap in a radial direction of said center electrode, wherein a size A of the first spark gap, which is a minimum distance between the end of said center electrode and the tip portion of said first ground electrode in the axial direction of said center electrode, is in a range of 0.4 to 1.0 mm; a wall thickness T of said insulator at the end of said insulator is in a range of 0.3 to 0.8 mm; a distance B, which is a minimum distance between said insulator and the tip portion of said second ground electrode, is in a range of 0.2 to 0.9 mm; and a distance C, which is a distance between an edge of the tip portion of said second ground electrode and the inner edge of the end of said insulator in the axial direction of said center electrode, is in a range of −1.0 to 0.5 mm, the edge of the tip portion of said second ground electrode facing the end of said metal shell, the distance C taking a positive value when the edge of the tip portion of said second ground electrode is closer to the end of said center electrode than the inner edge of the end of said insulator in the axial direction of said center electrode, wherein said second ground electrode comprises a base member and a discharge member, the discharge member is made of one of a noble metal and an alloy of the noble metal; and the discharge member has a length with a first end facing the outer side surface of said center electrode in the radial direction of said center electrode and a second end joined to the base member.