Patent ID: 8624474

Claim:
A spark plug comprising: a rod-like center electrode; a ceramic insulator electrically insulating an outer circumference of the center electrode; a metal shell fixed by caulking to an outer circumference of the ceramic insulator, the metal shell including a cylindrical groove that bulges outwardly in an outer circumference direction, said cylindrical groove having a first end and a second end, a polygonal tool engagement portion that adjoins said first end of the groove and bulges outwardly in the outer circumference direction with respect to the groove, and a trunk portion that adjoins said second end of the groove, and that bulges out in the outer circumference direction with respect to the groove; and a ground electrode joined to the metal shell, said ground electrode forming a spark gap with the center electrode, wherein an opposite side distance “S” between opposing two sides of the polygonal tool engagement portion is 12 mm or less, wherein the section modulus “Z1” satisfy a relationship Z1>=49 mm3, wherein a distance A, a distance B and the section modulus “Z2” satisfy the following relationship: if A>B, then Z2>=62 mm3, if A<=B, then Z2>=53 mm3, where “Z1” represents a section modulus in the first end of the groove, and where “Z2” represents a section modulus in the second end of the groove, where the distance “A” represents a distance along an axis of the spark plug from the first end to an outermost portion having a largest outer diameter in the groove, and where the distance “B” represents a distance along said axis from the second end to the outermost portion.