| 主权项 |
1. A spark plug, comprising:
a cylindrical insulator having an axial hole in an axis direction of the spark plug; a center electrode inserted in a front end side of the axial hole; and a cylindrical metal shell arranged around an outer circumferential surface of the insulator, the metal shell having a thread portion formed on an outer circumferential surface thereof for mounting of the spark plug and a step portion formed on an inner circumferential surface thereof, the insulator having an engagement portion held on the step portion of the metal shell via an annular plate packing and a leg portion located front of the engagement portion with a clearance left between an outer circumferential surface of the leg portion and the inner circumferential surface of the metal shell, wherein the thread portion has a thread diameter of M12 or smaller; and wherein, assuming that: L (mm) is a distance from a front end of a contact region of the plate packing with the metal shell toward the front in the axis direction; and A (mm) is a size of the clearance in a direction perpendicular to the axis direction, when the clearance has a site where the size A becomes 0.5 mm or smaller, the site of the clearance where the size A becomes 0.5 mm or smaller is located at or rear of a position of 2 mm from the front end of the contact region of the plate packing with the metal shell toward the front in the axis direction, and the spark plug satisfies a relationship of A≧L×0.2+0.2 (mm) within a range of 3.0≦L≦4.0. |
