Abstract: A spark plug includes an insulator having a through hole formed in the direction of an axis, a rod-shaped center electrode inserted in the through hole and extending in the direction of the axis, a metal shell disposed around an outer circumference of the insulator, and a ground electrode electrically conducted with the metal shell and adapted to define a gap between the ground electrode and the center electrode. A front end part of the insulator has a front end surface, an outer circumferential surface and a curved surface region formed between the front end surface and the outer circumferential surface. In a cross section including the axis, a front end of an inner circumferential surface of the metal shell faces the curved surface region in a direction perpendicular to the axis. The curved surface region has a curvature radius of 0.2 mm to 0.8 mm.

Abstract: A spark plug includes an insulator having a through hole formed in the direction of an axis, a rod-shaped center electrode inserted in the through hole and extending in the direction of the axis, a metal shell disposed around an outer circumference of the insulator, and a ground electrode electrically conducted with the metal shell and adapted to define a gap between the ground electrode and the center electrode. A front end part of the insulator has a front end surface, an outer circumferential surface and a curved surface region formed between the front end surface and the outer circumferential surface. In a cross section including the axis, a front end of an inner circumferential surface of the metal shell faces the curved surface region in a direction perpendicular to the axis. The curved surface region has a curvature radius of 0.2 mm to 0.8 mm.

Abstract: A spark plug includes: a rod-shaped center electrode extending in an axial direction; an insulator including a shaft hole that holds the center electrode inside of the shaft hole; a metal shell surrounding and holding a part of the insulator; and a ground electrode including a base end welded to the metal shell. The metal shell and the ground electrode are joined via a fusion portion formed by welding the ground electrode and the metal shell together. When the smallest thickness of the fusion portion in the axial direction is A, when, in a cross section that includes a center line of the ground electrode and is parallel to the axis, a length of a ground-electrode-side melted boundary formed between the fusion portion and the ground electrode is B, and when the ground electrode thickness is C, conditions of A?0.2 mm and B>C are satisfied.

Abstract: A spark plug provides a reduced production cost by forming a tip from a metallic material containing Ni that exhibits excellent erosion resistance. The spark plug comprises an insulator having an axial hole extending in an axial direction; a center electrode inserted into the axial hole at a front end thereof; a metallic shell provided around the insulator; a ground electrode whose proximal end portion is fixed to a front end portion of the metallic shell; and a tip bonded to a distal end portion of the ground electrode such that a spark discharge gap is formed between the tip and a front end portion of the center electrode. The tip is formed of a metallic material containing Ni in an amount of 93 mass % or more, and has a Vickers hardness of 163 Hv or less.

Abstract: A spark plug provides a reduced production cost by forming a tip from a metallic material containing Ni that exhibits excellent erosion resistance. The spark plug comprises an insulator having an axial hole extending in an axial direction; a center electrode inserted into the axial hole at a front end thereof; a metallic shell provided around the insulator; a ground electrode whose proximal end portion is fixed to a front end portion of the metallic shell; and a tip bonded to a distal end portion of the ground electrode such that a spark discharge gap is formed between the tip and a front end portion of the center electrode. The tip is formed of a metallic material containing Ni in an amount of 93 mass or more, and has a Vickers hardness of 163 Hv or less.