Patent ID: 7859178

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 and having a through-bore; a center electrode, having a leading end portion, which extends through the through-bore of the porcelain insulator and retained therewith in a fixed place along the central axis of the porcelain insulator; and a ground electrode extending from the metal shell and having a leading end placed in face-to-face relationship with the leading end portion of the center electrode to provide a spark discharge gap; wherein the through-bore of the porcelain insulator is exposed from an end face of the leading end portion of the ground electrode in a cross-sectional area S 1 , as viewed in a center axis direction of the center electrode and the porcelain insulator from a distal end of the spark plug, and has a total cross-sectional area S 2 falling in the relationship expressed as S 1 /S 2 ≦0.3; wherein the center electrode protrudes from an end face of the porcelain insulator with a protruding length L under the relationship expressed as L≦0.6 mm; wherein a minimal value Hmin and a maximal value Hmax in distance between a planar surface section of the ground electrode and the end face of the porcelain insulator lay in the relationship expressed as Hmax/Hmin≦1.3; wherein a wall thickness T between the through-bore and an outer periphery of the porcelain insulator lies in the relationship expressed as T≦0.7 mm; and wherein the leading end of the center electrode has a diameter d under the relationship expressed as d≦0.6 mm.