Abstract: The spark plug includes a cylindrical insulator, a center electrode provided in the insulator, and a cylindrical metal shell provided around the insulator. A ledge portion of the metal shell has a frontward facing surface facing a front side, a rearward facing surface facing a rear side, and a connection surface connecting the rearward facing surface and the frontward facing surface. The rearward facing surface engages with a step portion of the insulator. The metal shell includes a front cylindrical portion connected to the front side of the ledge portion. The inner circumferential surface of the front cylindrical portion is connected to the frontward facing surface of the ledge portion via a chamfered surface or a rounded surface. A corner at which the connection surface and the frontward facing surface are connected is located on the front side with respect to a front end of the insulator.

Abstract: The spark plug includes a cylindrical insulator, a center electrode provided in the insulator, and a cylindrical metal shell provided around the insulator. A ledge portion of the metal shell has a frontward facing surface facing a front side, a rearward facing surface facing a rear side, and a connection surface connecting the rearward facing surface and the frontward facing surface. The rearward facing surface engages with a step portion of the insulator. The metal shell includes a front cylindrical portion connected to the front side of the ledge portion. The inner circumferential surface of the front cylindrical portion is connected to the frontward facing surface of the ledge portion via a chamfered surface or a rounded surface. A corner at which the connection surface and the frontward facing surface are connected is located on the front side with respect to a front end of the insulator.

Abstract: A spark plug includes a tubular metal shell that includes a metal-shell step portion extending in an inner circumferential direction, an insulator that is inserted in the metal shell, that includes a facing portion that faces the metal-shell step portion with an annular packing interposed therebetween, a center electrode that extends in the axial direction, that has a flange portion extending in an outer circumferential direction, and a seal body seals the insulator and the center electrode. In a section that contains the axial line and that is along the axial line, a distance L along the axial line from a rear end of the facing portion of the insulator to a rear end of a portion at which the flange portion is in contact with the insulator satisfies L?1.1 (mm).

Abstract: A spark plug includes a tubular metal shell that includes a metal-shell step portion extending in an inner circumferential direction, an insulator that is inserted in the metal shell, that includes a facing portion that faces the metal-shell step portion with an annular packing interposed therebetween, a center electrode that extends in the axial direction, that has a flange portion extending in an outer circumferential direction, and a seal body seals the insulator and the center electrode. In a section that contains the axial line and that is along the axial line, a distance L along the axial line from a rear end of the facing portion of the insulator to a rear end of a portion at which the flange portion is in contact with the insulator satisfies L?1.1 (mm).

Abstract: A spark plug includes an insulator having an axial hole formed in a direction of an axis, a center electrode held in one end side of the axial hole, a metal terminal held in the other end side of the axial hole, an electrical connection part arranged to establish electrical connection between the center electrode and the metal terminal within the axial hole, and a metal shell disposed around an outer circumference of the insulator and having a thread portion formed on at least a part of an outer circumferential surface thereof. The electrical connection part has a resistor, and a conductive seal layer provided between the resistor and the center electrode to seal and fix the insulator and the center electrode together. In a half or more of a region in which the seal layer is provided in the direction of the axis, the spark plug satisfies predetermined conditions.

Abstract: A method for manufacturing a spark plug comprising the steps of: placing an assembly of the center electrode, an insulator, and a metal shell in a pressure resistant vessel and determining whether or not the insulator has a defect by applying a voltage to the center electrode, and bending the ground electrode toward the center electrode after the defect inspection step, the defect inspection step being performed while a pressure in the pressure resistant vessel is changed to a plurality of different pressures.

Abstract: A spark plug includes an insulator having an axial hole formed in a direction of an axis, a center electrode held in one end side of the axial hole, a metal terminal held in the other end side of the axial hole, an electrical connection part arranged to establish electrical connection between the center electrode and the metal terminal within the axial hole, and a metal shell disposed around an outer circumference of the insulator and having a thread portion formed on at least a part of an outer circumferential surface thereof. The electrical connection part has a resistor, and a conductive seal layer provided between the resistor and the center electrode to seal and fix the insulator and the center electrode together. In a half or more of a region in which the seal layer is provided in the direction of the axis, the spark plug satisfies predetermined conditions.

Abstract: It is an object to reduce eccentricity between a terminal nut and an insulator. A spark plug includes an insulator, a terminal nut, and a metallic shell. The outside diameter of the insulator at a rear end of the metallic shell is smaller than or equal to 8 mm, and the contact area between a flat portion of the insulator and a contact surface of the terminal nut is smaller than 10 mm2.

Abstract: A packing between an insulator and a metal shell of a spark plug. A length between a first position of a contact portion of the packing and the insulator and a second position of a contact portion of the packing and the insulator parallel to an axial line direction is set as a first length. In the case where a load perpendicular to the axial line direction is applied to the second position, a ratio of stress at a surface position that is a position on a surface of the insulator to stress at the first position is set as a stress ratio. In a range of the surface position where the stress ratio is 0.8 or more to 1.15 or less, a length in a continuous range from the first position toward a front end side parallel to the axial line direction is set as a second length. A ratio of the second length to the first length is 0.7 or more.

Abstract: A method for manufacturing a spark plug comprising the steps of: placing an assembly of the center electrode, an insulator, and a metal shell in a pressure resistant vessel and determining whether or not the insulator has a defect by applying a voltage to the center electrode, and bending the ground electrode toward the center electrode after the defect inspection step, the defect inspection step being performed while a pressure in the pressure resistant vessel is changed to a plurality of different pressures.

Abstract: A method for manufacturing a spark plug includes an assembling step of assembling an insulator into a metallic shell by insertion of the insulator into the metallic shell from an axially rear end opening portion of the metallic shell. The assembling step includes a displacement restricting step of restricting a relative positional displacement between the metallic shell and the insulator in a radial direction intersecting with the axial direction so as to reduce eccentricity between the axis of the metallic shell and the axis of the insulator to a predetermined value or less, while allowing a relative positional displacement between the metallic shell and the insulator in the axial direction. In the assembling step, whether or not the insulator is cracked in the assembling step is judged by detecting acoustic emission from the insulator.

Abstract: It is an object to reduce eccentricity between a terminal nut and an insulator. A spark plug includes an insulator, a terminal nut, and a metallic shell. The outside diameter of the insulator at a rear end of the metallic shell is smaller than or equal to 8 mm, and the contact area between a flat portion of the insulator and a contact surface of the terminal nut is smaller than 10 mm2.

Abstract: A packing is arranged between an outer-diameter-contracted portion of an insulator and an inner-diameter-contracted portion of a metal shell. In a contact portion of the packing and the insulator, a position at a most front end side is set as a first position. In a surface of a nose portion disposed at a front end side of the outer-diameter-contracted portion of the insulator, a position where a length from a front end of the insulator parallel to an axial line direction is 1 mm is set as a second position. A length between the first position and the second position parallel to the axial line direction is set as a first length. In the case where a load perpendicular to the axial line direction is applied to the second position, a ratio of stress at a surface position that is a position on a surface of the insulator to stress at the first position is set as a stress ratio. In a range of the surface position where the stress ratio is 0.8 or more to 1.

Abstract: A spark plug includes a center electrode, an insulator, a metallic shell, and a seal member for providing a seal between the insulator and the metallic shell. The insulator includes a first portion, a second portion located axially forward of the first portion and being smaller than the first portion, and an insulator first-diameter-reducing-portion whose outside diameter reduces forward and which connects the first portion and the second portion. The metallic shell includes a protrusion that includes a metallic shell diameter-reducing-portion whose inside diameter reduces forward. The seal member is disposed between the insulator first-diameter-reducing-portion and the metallic shell diameter-reducing-portion.

Abstract: A spark plug includes a ceramic insulator having an engagement portion, and a metallic shell provided around the ceramic insulator and having a protrusion. The protrusion has a diameter-decreasing portion, which seats on the engagement portion via an annular seat packing. On a cross section including an axial line, ?s>?p is satisfied, where ?p represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the engagement portion, and ?s represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the diameter-decreasing portion. In the aforementioned cross section, Hvo>Hvi is satisfied, where Hvo represents the Vickers hardness (Hv) of the seat packing at the midpoint of a first line segment, and Hvi represents the Vickers hardness (Hv) of the seat packing at the midpoint of a second line segment.

Abstract: A spark plug includes an insulator including an axial hole and having an outer periphery with a tapered outer face; a metal shell having a thread portion and a tapered inner face; and a circular packing sandwiched between the tapered outer face of the insulator and the tapered inner face of the metal shell. On at least one cross section including the axis, (A/B)?3.1, B?0.25, and (A+B)?2.0 are satisfied, where A represents a length of (a difference between an effective diameter of the thread portion and an inner diameter at a rear end of the tapered inner face)/2, and B represents a length of (a difference between the inner diameter at the rear end of the tapered inner face and an inner diameter at a front end of the tapered inner face)/2.

Abstract: A spark plug includes a ceramic insulator having an engagement portion, and a metallic shell provided around the ceramic insulator and having a protrusion. The protrusion has a diameter-decreasing portion, which seats on the engagement portion via an annular seat packing. On a cross section including an axial line, ?s>?p is satisfied, where ?p represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the engagement portion, and ?s represents an acute angle (°) between a straight line orthogonal to the axial line and the contour of the diameter-decreasing portion. In the aforementioned cross section, Hvo>Hvi is satisfied, where Hvo represents the Vickers hardness (Hv) of the seat packing at the midpoint of a first line segment, and Hvi represents the Vickers hardness (Hv) of the seat packing at the midpoint of a second line segment.

Abstract: A spark plug includes a center electrode, an insulator, a metallic shell, and a seal member for providing a seal between the insulator and the metallic shell. The insulator includes a first portion, a second portion located axially forward of the first portion and being smaller than the first portion, and an insulator first-diameter-reducing-portion whose outside diameter reduces forward and which connects the first portion and the second portion. The metallic shell includes a protrusion that includes a metallic shell diameter-reducing-portion whose inside diameter reduces forward. The seal member is disposed between the insulator first-diameter-reducing-portion and the metallic shell diameter-reducing-portion.

Abstract: A spark plug includes an insulator including an axial hole and having an outer periphery with a tapered outer face; a metal shell having a thread portion and a tapered inner face; and a circular packing sandwiched between the tapered outer face of the insulator and the tapered inner face of the metal shell. On at least one cross section including the axis, (A/B)?3.1, B?0.25, and (A+B)?2.0 are satisfied, where A represents a length of (a difference between an effective diameter of the thread portion and an inner diameter at a rear end of the tapered inner face)/2, and B represents a length of (a difference between the inner diameter at the rear end of the tapered inner face and an inner diameter at a front end of the tapered inner face)/2.

Abstract: A spark plug includes a ceramic insulator extending in the direction of an axis (CL1) and a metallic shell provided around the ceramic insulator. Talc is pressed charged between the ceramic insulator and the metallic shell and a crimp portion is formed for fixing the metallic shell and the ceramic insulator while the metallic shell having the ceramic insulator inserted thereinto is supported by a tubular receiving member. The receiving member is made movable in relation to a pressing jig in its radial direction. Thus, at the time of pressing of the talc, etc., the eccentricity between the center axis (CL2), (CL3) of the pressing jig and the axis (CL1) in the radial direction can be reduced, whereby the misalignment between the axis (CL1) and the center axis of the metallic shell or the like assembly error can be suppressed.