Patent ID: 7176608

Claim:
A spark plug comprising: a metal shell formed with a first bore and a second bore, smaller in diameter than the first bore, and an outer periphery formed with a mounting thread; a porcelain insulator fixedly secured to the metal shell such that one end of the porcelain insulator protrudes from one end of the metal shell and including a largest-diameter section, which is accommodated in the first bore of the metal shell, and a small-diameter section, having one end extending from the largest-diameter section and the other end closer to the one end of the porcelain insulator, which is accommodated in the second bore of the metal shell, the largest-diameter section and the small-diameter section being opposed through a gap to an inner wall of the metal shell which defines the first and second bores; a center electrode retained within the porcelain insulator with an end thereof located outside the porcelain insulator; and a ground electrode joined to the metal shell, the ground electrode having a portion facing the end of the center electrode through a spark gap; wherein the metal shell having an inner diameter D 1 at a portion of the inner wall to which the largest-diameter portion is opposed through the gap and an inner diameter D 2 at a portion of the inner wall to which the small-diameter portion is opposed through the gap, the inner diameters D 1 and D 2 meeting a relation of (D 1 −D 2 )/2 which is less than or equal to 1.8 mm.