| 摘要 |
PROBLEM TO BE SOLVED: To provide a spark plug with multipolarized outer electrodes having a superior stain resistance by setting both a projecting distance from a main fitting to a front end face of an insulator fitted into the main fitting and an interval between an inner circumferential part of a tip of each outer electrode and either the front end face of the insulator or its extension plane respectively within specific ranges. SOLUTION: This outer triple-pole spark plug A has a tube-like main fitting 1, an insulator 2 with an axial hole 22, a center electrode 3, and three outer electrodes 4. The insulator 2 is fitted into the main fitting 1 with its front end part (long leg part) 21 exposed not less than 2.5 mm. The center electrode 3 is fitted into the axial hole 22 of the insulator 2 with its igniting part 31 projected. Each of the outer electrodes 4 has a tip face 41 facing parallel to a side face of the igniting part 31, and a tip inner circumference part 42 disposed parallel to a front end face 23 of the insulator 2, while its back end 43 is welded to a front end part 11 of the main fitting 1. An interval (b) between the tip inner circumference part 42 of each the outer electrode 4 and the front end face 23 of the insulator 2 or its extension plane is set to 0.3 mm-1.2 mm. Accordingly, spark discharge runs on the front end face 23 of the insulator 2 to the outer electrodes 4 so as to self-clean the front end face 23 of the insulator 2.
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