| 摘要 |
A spark plug comprises an insulator and a metal shell. The insulator includes a nose length portion at the forward end, and a tapered portion, extending from the rear end of the nose length portion toward the rear end of the insulator, increased in diameter toward the rear end. The metal shell includes a shoulder portion, protruding inward and having a retaining surface by which the tapered portion is retained, and a male thread portion on the outer peripheral side of the shoulder portion, and the thread diameter of the male thread portion is set to M12 or less. When the insulator's cross-sectional area, perpendicular to an axis, passing through the boundary between the nose length section and tapered portion is B (mm2), and the metal shell's cross-sectional area, perpendicular to the axis, passing through the tip of the retaining surface is C (mm2), 2.80@C/B@3.50 is satisfied.
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