A spark plug including a cylindrical insulator having a step portion; a center electrode provided in an axial hole of the insulator; and a cylindrical metal shell having a ledge portion, the metal shell holding the insulator from an outer circumferential side in a state in which the step portion is engaged with the ledge portion via a packing, wherein a recess is formed on a part contacting with the packing, of one of the step portion and the ledge portion, and a projection which at least partially overlaps the recess in the axial-line direction is formed on a part contacting with the packing, of the other of the step portion and the ledge portion.
1. A spark plug comprising:
a cylindrical insulator having an axial hole extending in a direction of an axial line from a front side to a rear side, the insulator having, on an outer circumference thereof, a step portion of which an outer diameter reduces toward the front side in the axial-line direction;
a center electrode provided in the axial hole; and
a cylindrical metal shell having, on an inner circumference thereof, a ledge portion of which an inner diameter reduces toward the front side in the axial-line direction, the metal shell holding the insulator from an outer circumferential side in a state in which the step portion is engaged with the ledge portion via a packing, wherein
a recess is formed on a part contacting with the packing, of one of the step portion and the ledge portion, and
a projection which at least partially overlaps the recess in the axial-line direction is formed on a part contacting with the packing, of the other of the step portion and the ledge portion.
2. The spark plug according to claim 1 , wherein
a height of the projection is smaller than a depth of the recess.
3. The spark plug according to claim 1 , wherein
the recess is formed on the step portion, and
a minimum value of a radial-direction thickness of the insulator at a position of the recess is greater than a radial-direction thickness of the insulator on an inner side of a part closest to the axial line, of an inner circumference of the metal shell on the front side with respect to the recess.
4. The spark plug according to claim 1 , wherein
the recess is formed on the ledge portion.
5. The spark plug according to claim 4 , wherein
a center of the recess in a radial direction of the ledge portion is positioned on a radially outer side with respect to a center in the radial direction of the ledge portion.
6. The spark plug according to claim 1 , wherein
the packing includes a base material and a metal member sandwiched between the base material and the recess formed on one of the step portion and the ledge portion, and
Vickers hardness of the metal member is lower than Vickers hardness of the base material.
7. The spark plug according to claim 1 , wherein
the packing includes a base material and a metal layer formed on at least a part of a surface of the base material,
the metal layer is sandwiched between the base material and one of the step portion and the ledge portion on which the recess is formed, and
Vickers hardness of the metal layer is lower than Vickers hardness of the base material.
8. The spark plug according to claim 1 , wherein
an edge and a corner of each of the recess and the projection are chamfered to be rounded.