A spark plug is mounted to, for example, a combustion apparatus, such as an internal combustion engine, for igniting an air-fuel mixture contained in a combustion chamber. Generally, a spark plug includes an insulator having an axial bore; a center electrode inserted into a front end portion of the axial bore; a metallic shell provided externally of the outer circumference of the insulator; and a ground electrode provided at a front end portion of the metallic shell and forming a spark discharge gap in cooperation with the center electrode. Also, the metallic shell has a tool engagement portion for allowing a tool to be engaged therewith when the spark plug is to be mounted to the combustion apparatus.
A generally known tool engagement portion has a hexagonal cross section. However, in recent years, there is proposed a tool engagement portion having a 12-point shape (also called a “Bi-Hex shape”), in which a plurality of protrusions (ridges) and recesses (grooves) are provided alternately along its outer circumference (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-66385). As compared with a tool engagement portion having a hexagonal cross section, the tool engagement portion having a 12-point shape has the following merits.
In association with a recent demand for a reduction in the size of a spark plug, the diameter of the metallic shell is reduced. In view of retainment of strength or the like, the tool engagement portion must have a wall thickness of a certain minimum size. Thus, when the tool engagement portion has a hexagonal cross section, the inside diameter of the metallic shell must be sufficiently reduced. However, in association with a reduction in the inside diameter of the metallic shell, an insulator to be inserted into the metallic shell must be reduced in diameter. As a result, the insulator may deteriorate in dielectric strength and mechanical strength. By contrast, by means of the tool engagement portion having a 12-point shape, the metallic shell can be reduced in diameter without need to excessively reduce the inside diameter of the metallic shell, so that the tool engagement portion can retain a sufficient wall thickness. That is, by means of the tool engagement portion having a 12-point shape, while the spark plug is reduced in size, deterioration in dielectric strength and mechanical strength of the insulator can be effectively prevented.