Patent ID: 6628050
Filing Date: 2003-09-30
Classification: H01T

Abstract:
A spark plug comprising, in combination:a cylindrical metallic shell having a stepped portion on an inner wall thereof; an insulator disposed inside the metallic shell while being engaged with the stepped portion of the metallic shell, the insulator having an axially extending through hole and being formed such that the outer diameter of the insulator decreases toward the tip end side from an engagement position at which the insulator engages the stepped portion and such that the diameter decreases in a step at an axial position between the engagement position and the tip end of the insulator; a center electrode fixed within the through hole of the insulator such that a tip end portion of the center electrode projects from the tip end of the insulator or is located at the tip end; and a ground electrode having a base end portion connected to the tip end portion of the metallic shell and a tip end portion bent toward the center electrode to thereby form a spark discharge gap in cooperation with a side surface of the center electrode, wherein a diameter reduction ratio Y1=D1/d1 is 0.6 or less in a region of at least 2 mm extending from the tip end surface of the insulator toward the base end side, wherein D1 represents the outer diameter of the insulator measured at an arbitrarily determined axial position, and d1 represents the inner diameter of the tip end portion of the metallic shell.