| 摘要 |
A spark plug has a center electrode, a ceramic insulator having an axial hole to support the center electrode therein, a metal shell holding the ceramic insulator, and a ground electrode, one end portion of which is fixedly connected with the metal shell, the other end portion of which is located apart from an outer circumferential surface of a top end portion of the center electrode for defining a spark discharge gap therebetween. The ceramic insulator is provided with recesses at an edge portion between a top end surface of the ceramic insulator and an inner circumferential surface of the axial hole. When defining first and second imaginary circles with the axis being their respective centers as circles passing through portions of the recesses whose radial distances from the axis are a maximum and a minimum respectively, a difference of diameters of the first and second imaginary circles is 0.08 mm or less. |
