A spark plug includes a center electrode, a tubular metallic shell which holds the center electrode therein in an insulated state, and a ground electrode which faces a forward end portion of the center electrode. The metallic shell has a penetration hole which penetrates the metallic shell in a direction intersecting an axial direction. The ground electrode has a press-fitted portion inserted into the penetration hole and fixed to the metallic shell, a projecting portion projecting toward an inner side of the metallic shell, and an insertion portion disposed between the press-fitted portion and the projecting portion, inserted into the penetration hole, and forming a space between the insertion portion and an inner surface of the penetration hole. An angle α formed between the inner surface of the penetration hole and an outer surface of the insertion portion falls within a range of 0°<α<90°.

A spark plug that prevents a decrease in strength of a member to which a mark is attached while ensuring a readability of the mark, and a method of manufacturing the spark plug. The spark plug is configured to ignite an air-fuel mixture in an internal combustion engine. The spark plug includes: a mark formed of an oxide film generated on a surface of a metallic member or is formed of the metallic member and the oxide film; and a coating material covering the whole mark and allowing transmission of light.

A spark plug that prevents a decrease in strength of a member to which a mark is attached while ensuring a readability of the mark, and a method of manufacturing the spark plug. The spark plug is configured to ignite an air-fuel mixture in an internal combustion engine. The spark plug includes: a mark formed of an oxide film generated on a surface of a metallic member or is formed of the metallic member and the oxide film; and a coating material covering the whole mark and allowing transmission of light.

The spark plug includes: an insulator having an axial hole extending in an axial direction; a center electrode disposed at a front end in the axial direction of the axial hole and having a front end portion that protrudes from a front side of the axial hole; a tubular metal shell holding the insulator; and a ground electrode having one end portion that is fixed to a through hole provided in the metal shell, the ground electrode having another end portion that forms a discharge gap between the other end portion and the front end portion of the center electrode. The through hole extends toward the front side in the axial direction, in a direction from an outer peripheral surface toward an inner peripheral surface of the metal shell. The one end portion is located on a rear side relative to the other end portion in the axial direction.

The spark plug includes: an insulator having an axial hole extending in an axial direction; a center electrode disposed at a front end in the axial direction of the axial hole and having a front end portion that protrudes from a front side of the axial hole; a tubular metal shell holding the insulator; and a ground electrode having one end portion that is fixed to a through hole provided in the metal shell, the ground electrode having another end portion that forms a discharge gap between the other end portion and the front end portion of the center electrode. The through hole extends toward the front side in the axial direction, in a direction from an outer peripheral surface toward an inner peripheral surface of the metal shell. The one end portion is located on a rear side relative to the other end portion in the axial direction.

This spark plug inhibits excessive increase in the temperature of a ground electrode to inhibit occurrence of pre-ignition. The spark plug includes: an insulator in which an axial hole extending in an axial line direction is formed; a center electrode disposed at a front end in the axial line direction of the axial hole and having a front end portion that protrudes from a front side of the axial hole; a tubular metal shell holding the insulator; and a ground electrode having one end portion that is fixed to a through hole provided in the metal shell, the ground electrode further having another end portion that forms a discharge gap between the other end portion and the front end portion of the center electrode. The through hole extends toward the front side in the axial line direction, in a direction from an outer peripheral surface toward an inner peripheral surface of the metal shell. The one end portion is located on a rear side relative to the other end portion in the axial line direction.

This spark plug inhibits excessive increase in the temperature of a ground electrode to inhibit occurrence of pre-ignition. The spark plug includes: an insulator in which an axial hole extending in an axial line direction is formed; a center electrode disposed at a front end in the axial line direction of the axial hole and having a front end portion that protrudes from a front side of the axial hole; a tubular metal shell holding the insulator; and a ground electrode having one end portion that is fixed to a through hole provided in the metal shell, the ground electrode further having another end portion that forms a discharge gap between the other end portion and the front end portion of the center electrode. The through hole extends toward the front side in the axial line direction, in a direction from an outer peripheral surface toward an inner peripheral surface of the metal shell. The one end portion is located on a rear side relative to the other end portion in the axial line direction.

A metal shell for a spark plug is made from a steel material that has increased carbon content and, in some embodiments, boron as well. The steel material is well-suited for extrusion because of its ductility, while maintaining requisite strength. The spark plug shell may have a reduced outer diameter (OD.sub.HL) at a crimped hot lock region, such as the case when the shell is used in smaller diameter spark plugs, such as M8 and M10 plugs. According to a non-limiting example, the spark plug shell steel material comprises 0.20-0.55 wt % carbon, inclusive.

A spark plug including an insulator and a metallic member such as a metallic shell. In the spark plug, a mark such as a two-dimensional code is provided on the surface of at least one of the insulator and the metallic member, Also, a protective layer covers the mark. The protective layer is optically transparent and contains a fluorescent substance.

A spark plug including an insulator and a metallic member such as a metallic shell. In the spark plug, a mark such as a two-dimensional code is provided on the surface of at least one of the insulator and the metallic member, Also, a protective layer covers the mark. The protective layer is optically transparent and contains a fluorescent substance.