Spark Plug For Internal Combustion Engine

A spark plug used for internal combustion engines, comprising:

• a metal shell including a through hole that extends in an axial direction, said metal shell having a metal convex portion that projects radially inwardly toward the through hole, wherein the metal convex portion is comprised of 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 including an axial bore that extends in the axial direction, the insulator also including in its outer circumferential face a first insulator taper portion that is fixed by the convex rearward face of the metal convex portion, a base between the taper portions disposed on the front end side with respect to the first insulator taper portion and facing the convex inner circumferential face in a proximity status, a second insulator taper portion positioned on the front end side with respect to the base between the taper portions and having a contracted outer diameter toward the front end, and an insulator front end portion that extends from the front end of the second insulator taper portion toward the front end side 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 between the taper portions;
• a center electrode accommodated and held in the axial bore of the insulator;
• a ground electrode provided in a front end portion of the metal shell so that a front end portion of the ground electrode faces the front end face of the center electrode, and 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;
• and (2)
• 2≦XX≦4, (3)
• where “G” (mm) is the spark discharge gap,
• where “A” (mm) is an inner diameter of the through hole located on a front end side with respect to the convex forward face of the metal convex portion,
• where “B” (mm) is an outer diameter of a border between the second insulator taper portion and 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.