Patent ID: 7847473

Claim:
A spark plug comprising: a rod-shaped center electrode having an electrode for spark discharge at a front end thereof; an insulator which has an axial hole extending along the direction of an axis of the center electrode and which holds the center electrode in the axial hole; a metallic shell surrounding the insulator, the metallic shell holding a trunk portion of the insulator by a crimp portion provided at a rear end of the metallic shell in a state in which the insulator engages a step portion formed on an inner circumferential surface of the metallic shell; and an annular first packing disposed between an inner circumferential surface of the crimp portion and an outer circumferential surface of the trunk portion, wherein the following relationship is satisfied: ( G+L )/2 ≧d>L where G represents an outer diameter in millimeters of the first packing, L represents an inner diameter in millimeters of the first packing, d represents an inner diameter in millimeters of a minimum diameter portion of the crimp portion, and wherein the following relationship is satisfied: N<M where M represents a hardness in Hv of the crimp portion, and N represents a hardness in Hv of the first packing, wherein the metallic shell has a tool engagement portion on an outer circumferential surface thereof, and the relationship E−G≧0.1 (mm) is satisfied, where E represents a diameter in millimeters of a portion of the inner circumferential surface of the metallic shell corresponding to the tool engagement portion.