| 摘要 |
A spark plug for internal combustion engine includes a central electrode having a flange portion extending in the axial direction and having, at least internally, a core member with a thermal expansion coefficient larger than that at its tip portion and located on its proximal end side to project in the radial direction, and a columnar portion of smaller diameter than the flange portion located closer to the distal end side than the flange portion; an insulator having an axial bore extending in the axial direction and holding the flange portion firmly in the axial bore and holding the columnar portion idly; and a main metal fittings for containing the insulator. Assuming the difference between the outside diameter of the columnar portion at an arbitrary position B in the axial direction and the inside diameter of the axial bore is Cb, and the difference between the outside diameter of the columnar portion at a position F in the axial direction closer to the distal end side than the position B in the axial direction and the inside diameter of the axial bore is Cf, a relation: Cb<Cf is satisfied. |
