A spark plug includes a housing, a porcelain insulator, a center electrode, a ground electrode, and a prechamber-defining portion. The prechamber-defining portion has a prechamber formed therein. The prechamber-defining portion has a plurality of spray holes. At least one of the spray holes is an upstream spray hole which is arranged upstream of a plug center axis in a gas flow within a main combustion chamber and designed such that and an angle which an extension in an opening direction makes with an inner wall surface of the prechamber is selected to be larger than 90°. A spark gap is formed upstream of a plug center axis in the gas flow within the main combustion chamber.

A spark plug for an internal combustion engine includes a cylindrical insulator, a center electrode, a cylindrical housing, and a plug cover. On a tip side of the insulator, an auxiliary combustion chamber is formed which is surrounded by at least the plug cover. The plug cover is provided with nozzle holes that communicate between the auxiliary combustion chamber and the outside of the auxiliary combustion chamber. A central axis of the nozzle hole is inclined with respect to a plug radial direction when viewed in a plug axial direction. At least a part around an outer opening of the nozzle hole is provided with a concave portion that is adjacent to the outer opening. The concave portion is recessed from the outside to the inside of the plug cover.

A spark plug includes a first discharge chip and a second discharge chip facing the first discharge chip to define a spark gap therebetween. The spark gap has a size or distance between the first and second discharge chips which is 0.23 mm or more and 0.6 mm or less. At least one of the first and second discharge chips is made from material mainly containing iridium or platinum and an additive of tantalum of 0.5 Wt % to 5.0 Wt %. This minimizes a risk that a portion of the first or second discharge chip may be separated or such a separated portion may be redeposited on the first or second discharge portion.

A spark plug and method of manufacturing, where the spark plug meets particular geometric relationships to maintain and potentially improve dielectric performance while downsizing other plug dimensions. The spark plug includes an insulator that can withstand higher voltages while having areas with a reduced cross-sectional thickness. In some embodiments, the insulator has a dielectric strength of 42 kV/mm or more with a radial thickness at the internal seal of 1.5 to 1.6 mm, inclusive, and a radial thickness at a gasket of 0.6 to 0.9 mm, inclusive.

A method of producing a cylindrical insulator for a spark plug includes a molding step of forming ceramic powder filled in a cavity defined by a mold and a molding pin into a compact. In a first removal step of removing the compact from the mold, the compact has at least one protrusion formed in at least one recess of the molding pin formed in an outer cylindrical surface of the molding pin, and the at least one protrusion is locked in the at least one recess, thereby allowing the compact to be removed with the molding pin from the mold. In a second removal step of removing the molding pin from the compact, the molding pin is turned or rotated in the circumferential direction about the compact, causing the at least one recess to cut the at least one protrusion from the compact, and thereafter the molding pin is removed from the compact.

A method of producing a cylindrical insulator for a spark plug includes a molding step of forming ceramic powder filled in a cavity defined by a mold and a molding pin into a compact. In a first removal step of removing the compact from the mold, the compact has at least one protrusion formed in at least one recess of the molding pin formed in an outer cylindrical surface of the molding pin, and the at least one protrusion is locked in the at least one recess, thereby allowing the compact to be removed with the molding pin from the mold. In a second removal step of removing the molding pin from the compact, the molding pin is turned or rotated in the circumferential direction about the compact, causing the at least one recess to cut the at least one protrusion from the compact, and thereafter the molding pin is removed from the compact.

This spark plug includes a cap portion covering a center electrode and an end of a ground electrode from a front side. In a cross section including an axial line, where a pitch of an external thread of a metal shell is X (mm), an axial-line-direction distance between a crest of a rear-end ridge of a full thread portion of an external thread and a rear end of a contact portion of an insulator with which the metal shell contacts directly or via another member is A (mm), an axial-line-direction distance between the rear end of the contact portion and a front end of the insulator is B (mm), and an axial-line-direction distance between the rear end of the contact portion and a seating surface of the metal shell is C (mm), 0<A<4×, B≤15, and C≤3.6 are satisfied.

A spark plug has an insulator having a through hole formed along an axial direction, a center electrode which is partially inserted into a portion of the through hole on a forward end side in the axial direction, and a glass seal portion which is in contact with the insulator and the center electrode within the through hole, in which the glass seal portion contains glass and an electrically conductive substance. The glass contains an Si component and a B component in a total amount of 50 mass % or more, as reduced to SiO.sub.2 and B.sub.2O.sub.3, a Zn component in an amount of 20 mass % to 35 mass % as reduced to ZnO, and an alkali metal component. The glass contains, as the alkali metal component, an Na component in an amount less than 1 mass % as reduced to Na.sub.2O.

A spark plug includes a metallic shell which holds a center electrode in an insulated condition, and a ground electrode is held in a penetration hole of the metallic shell. The penetration hole includes a circular counterbore portion, and a penetrating portion extending from the counterbore portion to an inner circumferential surface of the metallic shell. The ground electrode includes a fixing portion which is fixed to the counterbore portion, and an extension portion extending from the fixing portion. The extension portion has a flat surface which faces a forward end surface of the center electrode in the axial direction. The penetrating portion restricts the extension portion such that the flat surface of the extension portion faces toward a rear end side in the axial direction.

A spark plug includes a central electrode member and an outer electrode member. The central electrode member includes a central base and a plurality of electrode prongs extending in an axial direction from the central base. The outer electrode member surrounds the central electrode member. The outer electrode member includes a wall that is radially spaced from the plurality of electrode prongs to allow a series of electric arcs to form between the wall and the plurality of electrode prongs. The outer electrode member and the central electrode member are sized and positioned relative to one another such that a first rate of wear of the outer electrode member, along a longitudinal axis of the spark plug, is substantially equal to a second rate of wear of the central electrode member along the longitudinal axis.