Ngk Spark Plug Company, Limited

Shota Takaenoki

Haruki Yoshida

Kengo Fujimura

Hiroki Shimada

Seiji Nakano

A spark plug which includes an insulator having an axial hole extending along an axial line, a metallic shell disposed around the insulator, and a center electrode disposed in the axial hole to be located on the forward end side of the axial line is provided. The insulator has a step portion provided on the outer circumferential surface thereof and facing toward the forward end side. The metallic shell has a receiving surface provided on the inner circumferential surface thereof and facing toward the rear end side and with which the step portion is engaged via a packing. A portion of the step portion on the outer circumferential side is a first curved surface being convex toward the forward end side, and a portion of the step portion on the inner circumferential side of the first curved surface is a second curved surface being convex toward the rear end side.

Spark plug

To prevent a sealant from intruding between a flange and a stepped portion during manufacture of a spark plug and to prevent cracking of an insulator during use of the spark plug, a spark plug includes a center electrode including a flange, an insulator having a through-hole extending in an axial-line direction and holding the center electrode, and a sealant filled in the through-hole to fix the flange and the insulator to each other; the insulator including a stepped portion in which the through-hole reduces a diameter toward a front end side and that supports the flange, and a small-diameter portion that is continuous with a front end side of the stepped portion and in which the diameter of the through-hole is smaller than in the stepped portion; an angle &#x3b8;formed by the stepped portion and a plane perpendicular to the axial line and an angle &#x3b8;formed by the plane perpendicular to the axial line and an opposing surface of the flange opposing the stepped portion satisfies &#x3b8;&#x2265;6&#xb0;, and in a cross section including the axial line, a maximum diameter Dof the flange and a diameter Dof the through-hole at a rear end of the small-diameter portion in the axial-line direction satisfy 0.15 mm&#x2264;(D&#x2212;D)/2.

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Nagoya, Japan

Shota Takaenoki