Patent ID: 7923910

Claim:
A spark plug comprising: a center electrode extending in an axial direction; an insulator which holds an outer circumference of the center electrode; a metallic shell which holds an outer circumference of the insulator; and a ground electrode joined to a front end portion of the metallic shell and forming a gap in cooperation with the center electrode; wherein the metallic shell has an outer surface having: a taper portion which comes into contact with a peripheral region around a mounting hole of an internal combustion engine when the spark plug is mounted in the mounting hole; a tool engagement portion with which a tool is engaged when mounting the spark plug into the mounting hole; and a trunk portion formed between the tool engagement portion and the taper portion, and an inner surface having: an annular step portion projecting toward the insulator and an internal trunk portion extending from a base of the step portion toward a rear end of the metallic shell, wherein an outside diameter of the trunk portion is represented by B, a minimal outside diameter of the taper portion is represented by C, an inside diameter of the internal trunk portion is represented by D, and an inside diameter of the step portion is represented by E, the insulator and the metallic shell are fixed to each other by cold crimping, and a projected area (π(B/2) 2 −π(C/2) 2 ) of the taper portion, defined as a difference between an area of a region surrounded by an outline of the trunk portion projected on an imaginary plane orthogonal to the axis and an area of a region surrounded by an outline of the taper portion at its minimal outside diameter projected on the imaginary plane, is at least two times a projected area (π(D/2) 2 −π(E/2) 2 ) of the step portion defined as a difference between an area of a region surrounded by an outline of the internal trunk portion projected on the imaginary plane and an area of a region surrounded by an outline of the step portion projected on the imaginary plane.