Patent ID: 8729785
Filing Date: 2014-05-20
Classification: H01T

Abstract:
1. A spark plug, comprising: an insulator having an axial hole extending in a direction of an axis; a center electrode inserted in a forward end portion of the axial hole; and a hollow cylindrical metal shell disposed on the outer periphery of the insulator, wherein the insulator includes an insulator nose length portion provided at the forward end of the insulator, and a tapered portion, extending from a rear end of the insulator nose length portion toward a rear end of the insulator in the axis direction, said tapered portion having a diameter increasing toward the rear end of the insulator in the axis direction, and the metal shell includes a shoulder portion, protruding inward in a radial direction, having a retaining surface by which the tapered portion is directly or indirectly retained, and a male thread portion, positioned on the outer peripheral side of the shoulder portion, for bringing the spark plug into threaded engagement with a mounting hole of a combustion device, wherein the thread diameter of the male thread portion is M when the area of a cross section of the insulator, perpendicular to the axis, passing through a boundary between the insulator nose length portion and the tapered portion is B (mm the area of a cross section of the metal shell, perpendicular to the axis, passing through a forward end of the retaining surface is C (mm 2.80≦C/B≦3.50 is satisfied.