| 摘要 |
A spark plug includes an outer electrode including an outer electrode tip of which a distal end surface is spaced from an outer peripheral surface of a leading end portion of a center electrode to define a spark discharge gap. A protruding insulator portion of a cylindrical insulator protrudes at least 1.0 mm from a leading end surface of a cylindrical metal shell. A protruding center electrode portion of the center electrode protrudes at least 3.5 mm from the leading end surface of the cylindrical metal shell. A relationship (theta1+theta2)/2>=75 degrees is satisfied where the angle theta1 is defined as a central angle (degrees) of a first circular sector having a point B1 as a center thereof, and the angle theta2 is an average value (degrees) of the central angles of two second circular sectors having the point B1 as a center thereof.
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