| 摘要 |
PROBLEM TO BE SOLVED: To provide a spark plug having excellent cold starting, heat-resistant and pollution-resistant property, and there is hardly an occurrence of bridge. SOLUTION: A first reduced diameter portion 3e1 and a second reduced diameter portion 3e2 are provided at an insulator 3 for forming step therein, a large space can be formed between the insulator and a main metal member 5 for preventing a starting error due to an occurrence of bridge in cold starting. Also, the diameter-reducing rate Y1 is set <=60% in the range of 2 mm toward a base end from a leading end of the insulator, and a large space can be formed between the insulator and an earth electrode 4 for increasing cold starting effect by intake of flashed mixed gas and for increasing heat-resistant property by suppressing the temperature of the leading end. Further, an outer diameter of the insulator is reduced rapidly between the first the second reduced diameter portions to form the step between them, therefore an electric field strength is stronger than that of the other. Although a crossly flying flame is generated, flying flame is not generated at the base end of the main metal member, as the flying is concentrated on the step. Accordingly, a self-cleaning effect can be enhanced.
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