Patent ID: 8120235

Claim:
A spark plug for an internal combustion engine comprising: a rod-shaped center electrode extending in a direction of an axial line; a substantially-cylindrical insulator provided around an outer periphery of the center electrode; a substantially-cylindrical metal shell provided around an outer periphery of the insulator; a ground electrode arranged to extend from a leading end portion of the metal shell such that a distal end of the ground electrode is bent toward the center electrode; a center electrode noble metal tip joined to a leading end of the center electrode; and a ground electrode noble metal tip joined to a distal end side of a center-electrode-side side surface portion, of side surface portions of the ground electrode, located on a center electrode side, a distal end portion of the ground electrode noble metal tip projecting from a distal end portion of the ground electrode toward the axial line, and a side surface portion on a center electrode side of the ground electrode noble metal tip projecting from the center-electrode-side side surface portion of the ground electrode, wherein a gap is provided between the center electrode noble metal tip and the ground electrode noble metal tip, and the ground electrode noble metal tip overlaps in plan view the center electrode noble metal tip along the direction of the axial line, wherein a tip distal end projection length that is a shortest distance from the distal end portion of the ground electrode to the distal end portion of the ground electrode noble metal tip is 0.5 mm to 1.5 mm, wherein a tip side surface projection length that is a shortest distance from the center-electrode-side side surface portion of the ground electrode to the side surface portion on the center electrode side of the ground electrode noble metal tip is not less than 0.15 mm but not more than 0.6 mm; wherein the ground electrode has a back-side side surface portion situated on back of the center-electrode-side side surface portion, and a side surface portion situated between the center-electrode-side side surface portion and the back-side side surface portion, and at least one of a chamfered portion and a curved surface portion formed between adjacent side surface portions of the respective side surface portions, wherein, in a cross section perpendicular to a center axis of the ground electrode, a thicknesswise length of the chamfered portion along a thickness direction of the ground electrode is 0.2 mm or more, and a widthwise length of the chamfered portion along a widthwise direction of the ground electrode is 0.2 mm or more, and a curvature radius of the curved surface portion in the cross section perpendicular to the center axis of the ground electrode is 0.2 mm or more.