Patent ID: 8188642

Claim:
A spark plug used for internal combustion engines, comprising: a metal shell having a front end, a rear end and a through hole that extends from said front end to said rear end in an axial direction along an axis of said through hole, said metal shell having a metal convex portion that projects radially inwardly toward the axis of the through hole, the metal convex portion having a convex inner circumferential face having a smallest inner diameter portion, a convex rearward face positioned on the rear end side of the convex inner circumferential face, and a convex forward face positioned on the front end side of the convex inner circumferential face; an insulator having a front end, a rear end and an axial bore that extends in the axial direction, the insulator having an outer circumferential face, said circumferential face having a first insulator taper portion that is fixed relative to said metal shell by the convex rearward face of the metal convex portion, a base portion disposed toward the front end side of the insulator relative to the first insulator taper portion and adjacent the convex inner circumferential face of said metal shell, a second insulator taper portion disposed toward the front end side of the insulator relative to the base said second insulator taper portion having an outer diameter that contracts toward the front end of said insulator, and an insulator front end portion that extends from the second insulator taper portion toward the front end side of said insulator and has a uniform outer diameter or an outer diameter smaller than that of the front end of the second insulator taper portion, wherein a very-end portion of the convex inner circumferential face of the metal convex portion is held by the metal shell so as to face the base portion of said insulator; a center electrode disposed and held in the axial bore of the insulator; a ground electrode provided on a front end portion of the metal shell wherein a front end portion of the ground electrode faces the front end face of the center electrode, said front end portion of said ground electrode forming a spark discharge gap with a front end portion of the center electrode, wherein the spark plug satisfies the following representations (1) to (3), G ≦( A−B )/2; (1) A≧ 7.3; (2) and 2 ≦XX≦ 4, (3) where “G” (mm) is the spark discharge gap, where “A” (mm) is an inner diameter of the through hole of the metal shell taken at a location on a front end side relative to the convex forward face of the metal convex portion, where “B” (mm) is an outer diameter taken where the second insulator taper portion communicates with the insulator front end portion, and where “XX” (mm) is a length in the axial direction from a very-end portion of the convex inner circumferential face to the border between the second insulator taper portion and the insulator front end portion.