| 摘要 |
To improve heat conduction and anti-fouling performance of a spark plug used for combustion engines. A spark plug 1 includes an insulator 2, a metal shell 3, a center electrode 5 and a ground electrode 27. A spark discharge gap 33 is formed between a front end portion 28 of the center electrode 5 and the ground electrode 27. The metal shell 3 has a through hole 29 therein and a metal convex portion 21 inwardly radially projecting in the through hole 29. The metal convex portion 21 includes a convex rearward face 30, a convex inner circumferential face 31 and a convex forward face 32. The through hole 29 has an inner diameter A (mm) at a front end side inner circumferential face 40, which is located at the front end side with respect to the convex forward face 32. The insulator 2 is inserted in the through hole 29 and has a first insulator taper portion 14, a second insulator taper portion 36 and a base 37 between the taper portions. The present invention satisfies the following representations: G ‰¤ (A-B)/2; A ‰¥ 7.3; and 2 ‰¤ XX ‰¤ 4, where "B"(mm)is an outer diameter of a border K between an insulator front end portion 38 and the second insulator taper portion 36, where "G"(mm) is a distance of a spark discharge gap 33, and where "XX" (mm) is a length from a very-end portion FF of the convex inner circumferential face 31 to the border K in the axial C1 direction. |
