| 摘要 |
The spark plug has a top end portion of a main metal shell that is almost cylindrical on an outer circumferential face without having a plug attaching external thread screwed into the cylinder head. Further, the plug hole formed in the cylinder head has a top end counterpart corresponding to the top end portion of the main metal shell. And in the mounting structure of the spark plug into the cylinder head (plug hole), the clearance amount (phiD-phid) satisfies a relationwhere the outer diameter of the top end portion of the main metal shell is phid and the hole diameter of the top end counterpart of the plug hole is phiD.
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