In general, the spark plug is fitted to the internal combustion engine by screwing a screw thread formed on an outer periphery of a metal shell with a female screw formed in the mounting hole of the internal combustion engine. An annular seal member (gasket) is mounted on an outer periphery of the metal shell to prevent airtight leakage within a combustion chamber through the mounting hole. A general gasket is prepared by folding a metal plate, which is annularly formed, in a thickness direction, for example, so that a cross section (cross section orthogonal to a circumferential direction after formation) is S-shaped. When the spark plug is attached, the gasket is interposed and compressed between a protrusion part of the metal shell and an opening periphery of the mounting hole. The gasket is deformed with screwing, and an adhesion with and an axial force (reactive force exerted in an axial direction due to compression caused by fastening) on each of the protrusion part and the opening periphery are enhanced to seal in the airtight leakage.
In recent years, the internal combustion engines are downsized, and the performance is enhanced with a tendency to intensify the vibrations of the engine. Further, there is a tendency to raise a temperature within the combustion chamber. In the gasket, when the axial force is reduced by deformation caused by the vibration of the engine, or creep deformation occurring due to heating and cooling cycles associated with drive and stop to slacken the screwing, the adhesion may be deteriorated. Under the circumstances, there has been known a spark plug that ensures deformation at the time of mounting, and maintains the adhesion by defining a shape (folding back) of the gasket while enhancing a material strength of the gasket and suppressing a plastic deformation after fastening to ensure the axial force (for example, refer to Japanese Patent Unexamined Publication Application No. 2004-134120 “Patent Literature 1”),
In a direct-injection engine, a positional relationship among a ground electrode protruded toward an interior of the combustion chamber, a fuel injection port, and a spark discharge gap influences ignitability. Hence, it is desirable that a direction (the above positional relationship within the combustion engine) of the ground electrode can be freely adjusted due to rotation of a screw when the spark plug is fitted to the engine. Under the circumstances, there has been known that thread fastening when the spark plug is attached ensures a magnitude (variation in compression displacement) that enables the gasket to be crushed while maintaining the torque (for example, refer to Japanese Patent Unexamined Publication Application No. 2000-266186 “Patent Literature 2”). In Patent Literature 2, the variation in the compression displacement of the gasket is ensured by 0.5 mm or more, as a result of which the rotation of the screw in a state where a given axial force is maintained is ensured by 0.5 to 1 rotation or more so that the direction of the ground electrode can be adjusted.