A pre-chamber spark plug includes a shell extending along a longitudinal axis and comprising an inner surface. The pre-chamber spark plug further includes an insulator including an outer surface. The pre-chamber spark plug further includes a pre-chamber cap connected to the shell. The pre-chamber cap and the shell together forms a pre-chamber. The pre-chamber spark plug further includes an annular reservoir defined between the outer surface of the insulator and the inner surface of the shell. The annular reservoir is spaced apart from the pre-chamber with respect to the longitudinal axis. The pre-chamber spark plug further includes a plurality of projections disposed between the annular reservoir and the pre-chamber with respect to the longitudinal axis. The plurality of projections at least partially define a plurality of flow passages therebetween.

A pre-chamber spark plug includes a shell extending along a longitudinal axis and comprising an inner surface. The pre-chamber spark plug further includes an insulator including an outer surface. The pre-chamber spark plug further includes a pre-chamber cap connected to the shell. The pre-chamber cap and the shell together forms a pre-chamber. The pre-chamber spark plug further includes an annular reservoir defined between the outer surface of the insulator and the inner surface of the shell. The annular reservoir is spaced apart from the pre-chamber with respect to the longitudinal axis. The pre-chamber spark plug further includes a plurality of projections disposed between the annular reservoir and the pre-chamber with respect to the longitudinal axis. The plurality of projections at least partially define a plurality of flow passages therebetween.

A spark plug includes a plug cover provided at a tip end portion of a housing to cover an auxiliary combustion chamber where an electrical discharge gap is located. The plug cover has a curved surface portion. The curved surface portion includes an outer curved surface convexly curved outward. The curved surface portion has an inclined injection hole formed therein to allow communication between the auxiliary combustion chamber and an outside. The inclined injection hole is opened to be inclined, relative to a plug axial direction, outward in a plug radial direction toward a tip end side. In a cross section including a central axis of the inclined injection hole and along the plug axial direction, a tangent line of a portion of the outer curved surface that overlaps an extended line of the central axis of the inclined injection hole is orthogonal to the extended line.

A spark plug includes a plug cover provided at a tip end portion of a housing to cover an auxiliary combustion chamber where an electrical discharge gap is located. The plug cover has a curved surface portion. The curved surface portion includes an outer curved surface convexly curved outward. The curved surface portion has an inclined injection hole formed therein to allow communication between the auxiliary combustion chamber and an outside. The inclined injection hole is opened to be inclined, relative to a plug axial direction, outward in a plug radial direction toward a tip end side. In a cross section including a central axis of the inclined injection hole and along the plug axial direction, a tangent line of a portion of the outer curved surface that overlaps an extended line of the central axis of the inclined injection hole is orthogonal to the extended line.

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 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 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 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 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 housing for a spark plug. The housing includes a bore along the longitudinal axis X of the housing, as the result of which the housing has an outer side and an inner side, and an electroplated or chemically applied nickel-containing protective layer situated on at least one portion of the outer side of the housing and a sealing layer situated on the nickel-containing protective layer. The sealing layer contains silicon. A first intermediate layer is applied between the housing and the nickel-containing protective layer and/or a second intermediate layer is applied between the nickel-containing protective layer and the sealing layer and/or a cover layer is applied on the sealing layer. The sealing layer may be free of chromium.