Patent ID: 8294347

Claim:
A spark plug comprising: a cylindrical metal shell; a cylindrical insulator which is fitted in the metal shell and includes a front end portion exposed from a front end portion of the metal shell; a center electrode which is disposed inside the insulator so that a front end portion of the center electrode is exposed from the front end portion of the insulator; a ground electrode which is connected to the metal shell at one end portion of the ground electrode and is disposed so as to confront the front end portion of the center electrode at the other end portion of the ground electrode, so as to form a spark discharge gap between the other end portion of the ground electrode and the front end portion of the center electrode; and a packing which is mounted between the metal shell and the insulator for establishing a gastight seal between the metal shell and the insulator, wherein the metal shell is formed with a ledge portion having inside diameter reduced gradually towards the front end portion so as to form a packing engagement plane, the insulator is formed with a step portion having outside diameter reduced gradually towards the front end portion so as to form a packing engagement plane which confronts the ledge portion, and the packing is disposed between the ledge portion and the step portion, wherein a distance between the ledge portion and the step portion gets narrower as the ledge portion and the step portion extend radially inwards, and wherein an angle formed by the ledge portion and the step portion is one degree or larger and ten degrees or smaller.