Abstract: Disclosed is spark plug including: an insulator with an axial hole; a center electrode disposed in a front end side of the axial hole; a cylindrical metal shell holding therein the insulator; and a ground electrode having a rod-shaped electrode base made of a Ni-based material and connected at a base end portion thereof to the metal shell and a tip made of a noble metal-based material and joined to a distal end portion of the electrode base. The distal end portion of the electrode base has a first surface facing a front end surface of the center electrode and second and third surfaces connected to the first surface. The tip includes a first tip member joined to the first surface and a second tip member joined to at least one of the second and third surfaces. The first and second tip members are separated apart from each other.

Abstract: Disclosed is spark plug including: an insulator with an axial hole; a center electrode disposed in a front end side of the axial hole; a cylindrical metal shell holding therein the insulator; and a ground electrode having a rod-shaped electrode base made of a Ni-based material and connected at a base end portion thereof to the metal shell and a tip made of a noble metal-based material and joined to a distal end portion of the electrode base. The distal end portion of the electrode base has a first surface facing a front end surface of the center electrode and second and third surfaces connected to the first surface. The tip includes a first tip member joined to the first surface and a second tip member joined to at least one of the second and third surfaces. The first and second tip members are separated apart from each other.

Abstract: A spark plug having an insulator that includes a fitted portion in contact with a rear end surface of a ledge portion of a metal shell. The insulator has a recessed portion recessed radially inward. The recessed portion is formed in an outer circumferential surface which is located on the rear side of the insulator relative to the fitted portion and which is disposed in the metal shell. The area of an outer circumferential surface of the insulator, from a front end of the recessed portion to a front end of the fitted portion, is larger than or equal to the area of a surface of the insulator which is exposed to combustion gas.

Abstract: A spark plug having an insulator that includes a fitted portion in contact with a rear end surface of a ledge portion of a metal shell. The insulator has a recessed portion recessed radially inward. The recessed portion is formed in an outer circumferential surface which is located on the rear side of the insulator relative to the fitted portion and which is disposed in the metal shell. The area of an outer circumferential surface of the insulator, from a front end of the recessed portion to a front end of the fitted portion, is larger than or equal to the area of a surface of the insulator which is exposed to combustion gas.

Abstract: A spark plug includes a central electrode and a noble metal tip, which are joined together with a fusion portion interposed therebetween. A tip-adjoining boundary of the fusion portion has a shape that curves convexly toward the fusion portion. A central-electrode-adjoining boundary of the fusion portion has a shape that curves convexly toward the central electrode. The outline of a portion of the fusion portion exposed to the outer surface has a shape that curves concavely into the inner side of the fusion portion.

Abstract: A spark plug includes a central electrode and a noble metal tip, which are joined together with a fusion portion interposed therebetween. A tip-adjoining boundary of the fusion portion has a shape that curves convexly toward the fusion portion. A central-electrode-adjoining boundary of the fusion portion has a shape that curves convexly toward the central electrode. The outline of a portion of the fusion portion exposed to the outer surface has a shape that curves concavely into the inner side of the fusion portion.