| 摘要 |
An object of the present invention is to provide a spark plug having excellent under-load life. The spark plug includes 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 satisfies the following conditions: (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) the 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 the relational expression 0.9 ‰¤ A/B ‰¤ 0.98; and (5) Vickers hardness of the insert portion measured at the center of a cross section of the insert portion cut along a direction orthogonal to the axial line is 150 Hv or more to 350 Hv or less. |
