| 摘要 |
PROBLEM TO BE SOLVED: To provide a small size spark plug of a screw diameter 12 mm in which the terminal fittings are hardly worn out and the insulating body, the terminal fittings and the center electrode are firmly jointed. SOLUTION: In the spark plug 100, the terminal fittings 13 and the center electrode 3 are fixed through a conductive seal material layer 16, 17 in the penetrating hole 6 formed in the axis O direction of the insulator 2. A mounting screw 7 having a nominal size of M12 is formed on the outer circumference of the front side of the main fittings 1. The Vickers hardness of the terminal part 13a protruded backward from the insulator 2 of the terminal fittings 13 is modulated at Hv 150 or more and 300 or less. The front end 13d of the terminal fittings 13 is embedded in the conductive seal material layer 17 and the smaller diameter portion 13c is formed in a manner extending toward the back of the front end 13d. The difference of the diameter between the outer diameter d1 at the smaller diameter portion 13c and the inner diameter D6 of the insulator 2 is modulated at 1.0 mm or more and 1.4 mm or less.
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