Patent ID: 8841825

Claim:
A spark plug for an internal combustion engine comprising: a hollow cylindrical housing with a top end facing a top of the spark plug; a cylindrical porcelain insulator which is retained in the cylindrical housing; a center electrode which is retained in the porcelain insulator with a top thereof exposed outside a top end of the porcelain insulator; and a ground electrode which is joined to the cylindrical housing so as to form a spark gap between itself and the top of the center electrode; wherein the hollow cylindrical housing includes an inward top end protrusion which extends inwardly from the top end of the cylindrical housing, the inward top end protrusion being at least partially located to be closer to the top of the spark plug than the top end of the porcelain insulator is in an axial direction of the spark plug; wherein a distance L between the inward top end protrusion and the center electrode and a size G of the spark gap in the axial direction of the spark plug are so selected as to meet a relation of G<L; wherein the hollow cylindrical housing includes a retaining portion which retains the cylindrical porcelain therein and the inward top end protrusion which extends from the retaining portion inwardly in a radial direction of the spark plug toward a side surface of the center electrode; and wherein the center electrode has a top end surface in an axial direction of the spark plug, the top end being located outside the inward top end protrusion in the axial direction of the spark plug.