Patent ID: 7714490

Claim:
A spark plug for an internal combustion engine, comprising: a metal shell having an outer periphery formed with a mounting thread; a porcelain insulator fixedly secured to the metal shell on a central axis thereof; a center electrode retained within the porcelain insulator along a central axis thereof; and a ground electrode extending from the metal shell and having a leading end placed in face-to-face relationship with the center electrode to provide a spark discharge gap; the center electrode including a center electrode base material body of a substantially columnar shape having a base material leading end portion, and a substantially square shaped rod-like Ir alloy tip bonded to the base material leading end portion of the center electrode base material body; and the base material leading end portion of the center electrode base material body having a columnar shape with a diameter D 2 smaller than a diameter D 1 of the center electrode base material body; wherein the square shaped rod-like Ir alloy tip has a square shape in cross section on a plane perpendicular to an axis of the Ir alloy tip with diagonal lines among which a long diagonal line is supposed to be a diameter A of a circumscribed circle C A of the Ir alloy tip and an inscribed circle C B is supposed to internally touch at least two sides of the square shape coaxially with the circumscribed circle C A of the Ir alloy tip and have a diameter B, with four diameters A, B, D 1 and D 2 lay in the relationship expressed as D 1 >A>D 2 >B; the substantially square shape of the Ir alloy tip in cross section has corners formed in curved portions, respectively, and straight portions interconnecting the curved portions to each other; and wherein each straight portion has a length L and the Ir alloy tip has a width W under the relationship expressed as 0.8×W≦L<W.