Patent ID: 8339021
Filing Date: 2012-12-25
Classification: H01T

Abstract:
1. A spark plug for an internal-combustion engine, comprising: an attachment fitting which provides a screw part to an outer circumference of the attachment fitting, an insulator held by the attachment fitting so that the insulator tip part projects; a center electrode held by the insulator so that the electrode tip part projects from the insulator tip part; and an earth electrode which forms a spark discharge gap between the center electrode and the earth electrode, wherein: the earth electrode has a convex part formed by projecting toward the center electrode a part of an opposed surface of the earth electrode, that faces the center electrode, and a concave part formed toward the opposed surface from a back surface of the earth electrode, that is the reverse side of the earth electrode with respect to the opposed surface, while the earth electrode is fixed to the attachment fitting; the convex part is disposed so that the extension of a shaft center of the convex part passes through the area in which the concave part is formed; the convex part has a top surface that is formed as a flat planar surface; and a relation of S