Patent ID: 9035541
Filing Date: 2015-05-19
Classification: H01T

Abstract:
1. A spark plug comprising: an insulator having an axial hole extending along an axial line; a metal terminal member having an insert portion accommodated in the axial hole and a depression formation zone existing on an outer circumferential surface of the insert portion and having a plurality of depressions; and a metallic shell accommodating a forward portion of the insulator therein to thereby hold the insulator, the spark plug being characterized by satisfying the following conditions to: (1) the insert portion has a length H of 35 mm or more along the axial line; (2) the depression formation zone has a length F of 13 mm or more along the axial line; (3) the insert portion has a smooth surface zone on its outer circumferential surface; (4) a ratio (A/B) between diameter A of a forward end of the insert portion and inside diameter B of the insulator measured at the forward end satisfies a relational expression 0.9≦A/B≦0.98; and (5) the insert portion has a Vickers hardness of 150 Hv or more to 350 Hv or less as measured at the center of a cross section of the insert portion cut along a direction orthogonal to the axial line.