In a cross section including a central axis, a value L/D obtained by dividing an axial length L of an overlap portion where a metal contact surface located on a metal shell and on which a packing is in contact with the metal shell overlaps a projection plane located on the metal shell and on which a contact surface located on an insulator and on which the packing is in contact with the insulator is projected in a direction orthogonal to the central axis by a difference D between a radius of an outer circumference of a tube portion at a connection position to a step portion and a radius of an outer circumference of a leg portion at a connection position to the step portion is 1.2 or more. This ensures the force of constraint in the radial direction provided by the packing on the insulator.

A spark plug having an insulator with a leg portion whose outer circumferential surface has an arithmetic mean roughness Ra of 0.5 μm or less. The leg portion has a first section, a second section, and a third section. The first section has an outer diameter that decreases toward the forward end of the first section. The second section is located forward of and adjacent to the first section and its outer diameter decreases toward the forward end of the second section. The outer circumferential surface of the second section is located inward of a first straight line passing through the base and forward ends of the outer circumferential surface of the first section. The third section is located forward of and adjacent to the second section. The outer diameter of the third section is equal to or less than a forward end outer diameter Ds of the second section over the entire third section.

A spark plug including a terminal end, a firing end, an axial centerline extending between the terminal end and the firing end, and an insulative core extending between the terminal end and the firing end. The insulative core includes a central bore coincident with the axial centerline extending through the insulative core, and an insulative nose extending along and symmetrically about the axial centerline from an end surface of the insulative core at the firing end to a transition location along an axial length of the insulative core, wherein a perimeter surface of the insulative nose has a concave curvilinear shape between the end surface and the transition location.

A spark plug including a terminal end, a firing end, an axial centerline extending between the terminal end and the firing end, and an insulative core extending between the terminal end and the firing end. The insulative core includes a central bore coincident with the axial centerline extending through the insulative core, and an insulative nose extending along and symmetrically about the axial centerline from an end surface of the insulative core at the firing end to a transition location along an axial length of the insulative core, wherein a perimeter surface of the insulative nose has a concave curvilinear shape between the end surface and the transition location.

A spark plug having a housing, an insulator situated in the housing, a center electrode situated in the insulator and a ground electrode situated on the housing, the ground electrode and the center electrode being situated toward each other in such a way that the ground electrode and the center electrode form a spark gap, and the center electrode rest with a center electrode head on a seat developed on an inner side of the insulator, and the center electrode having an electrode base body and a core situated in the electrode base body, the core being made from a material that has a higher heat conductivity than the material of the electrode base body, the electrode base body having a diameter that is not greater than 1.7 mm.

A spark plug having a housing, an insulator situated in the housing, a center electrode situated in the insulator and a ground electrode situated on the housing, the ground electrode and the center electrode being situated toward each other in such a way that the ground electrode and the center electrode form a spark gap, and the center electrode rest with a center electrode head on a seat developed on an inner side of the insulator, and the center electrode having an electrode base body and a core situated in the electrode base body, the core being made from a material that has a higher heat conductivity than the material of the electrode base body, the electrode base body having a diameter that is not greater than 1.7 mm.

A composite sparking component for a spark plug that has a thin precious metal layer with a series of grooves attached to an underlying base layer. The grooves allow the precious metal layer, and hence the entire composite sparking component, to be more easily bent or formed into a desired shape, while at the same time minimizing the amount of precious metal and providing enhanced sparking sites along the edges of the grooves. In one example, the composite sparking component is a sleeve-shaped component attached to a center electrode. In a different example, the composite sparking component is a ring-shaped component attached to a ground electrode. The precious metal layer may be bonded to the base layer in the form of a bimetal laminate structure, or the precious metal layer can be built on the base layer with the use of additive manufacturing, to cite several possibilities.

A composite sparking component for a spark plug that has a thin precious metal layer with a series of grooves attached to an underlying base layer. The grooves allow the precious metal layer, and hence the entire composite sparking component, to be more easily bent or formed into a desired shape, while at the same time minimizing the amount of precious metal and providing enhanced sparking sites along the edges of the grooves. In one example, the composite sparking component is a sleeve-shaped component attached to a center electrode. In a different example, the composite sparking component is a ring-shaped component attached to a ground electrode. The precious metal layer may be bonded to the base layer in the form of a bimetal laminate structure, or the precious metal layer can be built on the base layer with the use of additive manufacturing, to cite several possibilities.

An employed rear mold is comprised of a plurality of mold pieces. The plurality of mold pieces include a first mold piece which forms a first partial internal surface of the internal surface of the rear mold along the entire circumference of the internal surface, and a second mold piece which is located axially forward of the first mold piece and forms a second partial internal surface of the internal surface of the rear mold along the entire circumference of the internal surface. Releasing the molded body includes starting to move the first mold piece rearward in relation to a rear molded portion, and, subsequently to starting to move the first mold piece, starting to move the second mold piece rearward in relation to the rear molded portion, thereby disassembling the rear mold into the plurality of mold pieces.

A tip that contains a noble metal is joined to a joining surface of an electrode base material of a ground electrode via a welded portion. On cross-sections of the tip and the electrode base material in a longitudinal direction of the joining surface, the welded portion has a void above the joining surface, and a continuous distance of the welded portion on the joining surface is less than or equal to 0.5 mm, whereby thermal stress due to a difference in thermal expansion between the tip and the electrode base material can be reduced. A total of continuous distances of the welded portions on the joining surface, are 0.4 times to 0.8 times a length from an end of the tip to another end thereof, whereby joining strength of the welded portion can be assured. Peeling at the tip or falling-off of the tip due to thermal stress can be less likely to occur, whereby durability of the ground electrode can be improved.