Patent ID: 8476815

Claim:
A spark plug, comprising: a center electrode extending in an axial direction; an insulator having an axial bore extending in the axial direction and holding the center electrode in the axial bore at a front end side; a cylindrical metal shell radially surrounding the insulator so as to hold the insulator and having a front end constituent face at a front end side opening thereof, wherein the front end constituent face is comprised of a visible external surface as viewed from a front end side in the axial direction of the spark plug, and has a plurality of faces; and a ground electrode having (a) one end joined to at least one of the plurality of faces constituting the front end constituent face and (b) an other end bent toward an inner circumferential of the metal shell so as to form a spark discharge gap with a front end portion of the center electrode, the ground electrode further comprising (i) an extending portion extending in the axial direction from the one end to the other end and (ii) a bending portion between the extending portion and the other end, wherein a virtual sphere having a radius of 1.2 mm is neither in contact with the center electrode nor the insulator, and is assumed to be in contact with an inner face of the bending portion of the ground electrode that faces toward the center electrode, wherein the plurality of faces constituting the front end constituent face of the metal shell include a first face adjoining at least a part of an inner circumferential face of the metal shell in the axial direction, said first face being an inclined face having a diameter that increases from a rear end side of the spark plug to the front end side of the spark plug in the axial direction of the spark plug, and wherein, as viewed on a cross-sectional outline of the metal shell including the axis thereof, a length of the first face is the longest of the plurality of faces constituting the front end constituent face of the metal shell.