Spark plug for internal combustion engine and method for producing the spark plug

A method for producing a spark plug, said spark plug comprising:

• a rod-shaped center electrode extending in the direction of an axis line;
• a substantially cylindrical insulator provided on an outer periphery of the center electrode;
• a cylindrical metal shell provided on an outer periphery of the insulator;
• a ground electrode having a base end joined to a leading end portion of the metal shell and a distal end bent toward the axis line, the ground electrode comprising:
• a thick portion provided on a base end side, a thin portion provided on a distal end side, and a stepped portion provided on an inner peripheral surface between the thick portion and the thin portion;
• a noble metal tip joined to and partially embedded in the inner peripheral surface of a distal end portion of the ground electrode, the noble metal tip being disposed to form a gap between the noble metal tip and the leading end portion of the center electrode, said method comprising:
• bending the ground electrode toward the axis line;
• and
• joining the noble metal tip to the inner peripheral surface of the distal end portion of the ground electrode,
• wherein after bending the ground electrode toward the axis line, the noble metal tip is joined to the inner peripheral surface of the distal end portion of the ground electrode.

A spark plug for an internal combustion engine, said spark plug comprising:

• a rod-shaped center electrode extending in the direction of an axis line;
• a substantially cylindrical insulator provided on an outer periphery of the center electrode;
• a cylindrical metal shell provided on an outer periphery of the insulator;
• a ground electrode having a base end joined to a leading end portion of the metal shell and a distal end bent toward the axis line, the ground electrode comprising:
• a thick portion provided on a base end side,
• a thin portion provided on a distal end side, and
• a stepped portion provided on an inner peripheral surface between the thick portion and the thin portion;
• and
• a first noble metal tip joined to and partially embedded in the inner peripheral surface of the thin portion, the first noble metal tip being disposed to form a gap between the first noble metal tip and the leading end portion of the center electrode,
• wherein a relationship −0.750≦“length L2”−“length L1”≦0.250 is satisfied where, when viewed from a side surface of the ground electrode,
• A (mm) is a thickness of the thick portion of the ground electrode;
• L1 is a center line which is a curve shifted by [A/2] to the inner peripheral side from an outer peripheral line of the ground electrode;
• a first point “a” is an intersection of the center line Ll and distal end surface of the ground electrode;
• a second point “b” is an intersection of the center line Ll and a plane including the distal end surface of the metal shell;
• “length L1” (mm) is a length between the first point “a” and the second point “b” of the center line L1;
• C (mm) is a thickness of the distal end surface;
• a first curve L2 is a curve shifted by [C−A/2] to the inner peripheral side from the center line L1;
• a third point “c” is an intersection of the first curve L2 and the distal end surface;
• a fourth point “d” is an intersection between the first curve L2 and a plane including a leading end surface of the metal shell;
• and
• “length L2” (mm) is a length between the third point “c” and the fourth point “d” of the first curve L2.