Patent ID: 8624475

Claim:
A spark plug, comprising: a cylindrical insulator extending in the direction of an axis of the spark plug; an annular plate packing; and a cylindrical metal shell arranged circumferentially around the insulator, the insulator including a step portion formed on an outer circumferential surface thereof and having an outer diameter decreasing toward the front in the direction of the axis and a leg portion located front of the step portion and extending toward the front in the direction of the axis, the metal shell including a flat taper portion formed on an inner circumferential surface thereof and having an inner diameter decreasing toward the front in the direction of the axis, the insulator being fixed in the metal shell by crimping a rear end portion of the metal shell with the step portion of the insulator retained on the flat taper portion of the metal shell via the plate packing, the insulator further including a cylindrical middle body portion located rear of the step portion and extending in the direction of the axis, said middle body portion dimensioned to define a space that communicates with the plate packing between the cylindrical middle body portion of the insulator and the metal shell and the insulator still further including a curved surface portion formed in a concave shape on the outer circumferential surface thereof at a position between the step portion and the leg portion, said curved surface portion defining a middle region between front and rear ends thereof, 50% or more of an inner circumferential edge portion of the plate packing in a circumferential direction thereof being in contact with the insulator at a location in front of said middle region of the insulator.