| 摘要 |
A spark plug according to this invention includes an insulator having a center through-hole formed therein, a center electrode held in the center through-hole, a metallic shell holding the insulator by swaging, and a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode. The metallic shell has a male-threaded portion formed on the outer circumferential surface of a front end portion of the metallic shell, and a tightening portion formed on the outer circumferential surface of the metallic shell, located at a rear side with respect to the male-threaded portion. The tightening portion has a width-across-flats W of not greater than 14 mm. A cushion material is filled into a space defined by the metallic shell and an insulator to form a cushion-material charged portion. The cushion-material charged portion has an axial length L of from 0.5 mm to 10.0 mm inclusive and a thickness M of from 0.5 mm to 1.3 mm inclusive.
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