A spark plug includes: a center electrode; a ground electrode that is provided such that a gap for spark discharge is formed between the center electrode and the ground electrode; and a plug cover covering the center electrode and the ground electrode from a front side. The plug cover has a through hole, wherein the plug cover includes a diameter reduction portion formed in a range of 0.1 mm or less from an outer open end of the through hole in a direction along a central axis of the through hole and having a diameter gradually decreasing from the outer open end toward an inner open end of the through hole. A relationship of 0 mm<xy<0.2 mm is satisfied, where x is a diameter at the outer open end, and y is a diameter at an inner end of the diameter reduction portion.

An ignition coil includes a coil main body portion, a conductive member, and a protective portion. The coil main body portion generates a high voltage. The conductive member electrically connects the coil main body portion and a terminal metal fitting of a spark plug. The conductive member is arranged inside the protective portion. The protective portion has an electrically insulating property. The conductive member includes an elastic portion that elastically deforms in a longitudinal direction of the conductive member, and a conductive terminal that is arranged on a distal end side of the elastic portion. The conductive terminal has a distal end surface that includes a concave surface or a convex surface that abuts on the terminal metal fitting of the spark plug. A contact portion between the concave surface or the convex surface and the terminal metal fitting of the spark plug has an annular shape.

A spark plug includes a tubular metallic shell having an attachment screw portion having an external thread. The attachment screw portion has a pitch diameter maximum portion at which the external thread has a maximum pitch diameter, the pitch diameter maximum portion being located on a rear end side of a center of the attachment screw portion in a direction along an axial line of the metallic shell, and a pitch diameter local maximum portion at which the external thread has a locally maximum pitch diameter, the pitch diameter local maximum portion being located on a forward end side of the pitch diameter maximum portion in the direction along the axial line.

A spark plug includes a tubular metallic shell having an attachment screw portion having an external thread. The attachment screw portion has a pitch diameter maximum portion at which the external thread has a maximum pitch diameter, the pitch diameter maximum portion being located on a rear end side of a center of the attachment screw portion in a direction along an axial line of the metallic shell, and a pitch diameter local maximum portion at which the external thread has a locally maximum pitch diameter, the pitch diameter local maximum portion being located on a forward end side of the pitch diameter maximum portion in the direction along the axial line.

A spark plug includes a tubular metallic shell having an attachment screw portion having an external thread. The attachment screw portion has a pitch diameter maximum portion at which the external thread has a maximum pitch diameter, the pitch diameter maximum portion being located on a rear end side of a center of the attachment screw portion in a direction along an axial line of the metallic shell, and a pitch diameter local maximum portion at which the external thread has a locally maximum pitch diameter, the pitch diameter local maximum portion being located on a forward end side of the pitch diameter maximum portion in the direction along the axial line.

A spark plug includes a plug cap having a plurality of through holes. In a cross section including an axial line, the plug cap includes an enlarging portion in a portion of a region between a first imaginary straight line and a second imaginary straight line, the first imaginary straight line extending through a front end of the end portion of a ground electrode and being perpendicular to the axial line, the second imaginary straight line extending through a back end of one of inner open ends of the through holes that is closest to a front end of the spark plug and being perpendicular to the axial line. The enlarging portion being a portion in which a minimum distance from an outer surface to an inner surface of the plug cap increases with increasing distance in a direction from back to front.

A spark plug includes a plug cap having a plurality of through holes. In a cross section including an axial line, the plug cap includes an enlarging portion in a portion of a region between a first imaginary straight line and a second imaginary straight line, the first imaginary straight line extending through a front end of the end portion of a ground electrode and being perpendicular to the axial line, the second imaginary straight line extending through a back end of one of inner open ends of the through holes that is closest to a front end of the spark plug and being perpendicular to the axial line. The enlarging portion being a portion in which a minimum distance from an outer surface to an inner surface of the plug cap increases with increasing distance in a direction from back to front.

A spark plug that provides an increase in the combustion speed of an engine. The spark plug includes a center electrode, a ground electrode, an insulator, and a cover portion. In a cross-sectional area of a region surrounded by an inner wall surface of the cover portion in a cross section taken along a plane perpendicular to an axial line CX of the spark plug, the cover portion includes: a narrow portion positioned on the front end side with respect to the ground electrode in the direction of the axial line CX and narrowed such that the cross-sectional area is smallest; and a cover front portion positioned on the front end side of the narrow portion and having the cross-sectional area that is larger than the cross-sectional area of the narrow portion and that is largest at the front end of the cover front portion.

A spark plug that provides an increase in the combustion speed of an engine. The spark plug includes a center electrode, a ground electrode, an insulator, and a cover portion. In a cross-sectional area of a region surrounded by an inner wall surface of the cover portion in a cross section taken along a plane perpendicular to an axial line CX of the spark plug, the cover portion includes: a narrow portion positioned on the front end side with respect to the ground electrode in the direction of the axial line CX and narrowed such that the cross-sectional area is smallest; and a cover front portion positioned on the front end side of the narrow portion and having the cross-sectional area that is larger than the cross-sectional area of the narrow portion and that is largest at the front end of the cover front portion.

An ignition coil is described with a housing having a tubular housing part for connecting to a spark plug, and a pin-shaped interference suppression resistor arranged in the tubular housing part. According to this disclosure, it is provided that the housing surrounds an interior which has a funnel-shaped section which tapers toward the tubular housing part. In addition, a method is described for producing such an ignition coil.