Patent ID: 7449824

Claim:
A spark plug, comprising: a center electrode; a ceramic insulator being formed with an axial through-hole to support therein the center electrode and including a front portion with a stepped outer surface, a middle portion made larger in outer diameter than the front portion, a rear portion made smaller in outer diameter than the middle portion and a shoulder portion defined between the middle and rear portions, a difference between the outer diameters of the middle and rear portions of the ceramic insulator being 1.8 mm or smaller; a metal shell being formed with an axial through-hole to hold therein the ceramic insulator and including a tool engagement portion adapted to engage with a plug mounting tool, a radially inward protrusion formed in the axial through-hole of the metal shell to retain thereon the stepped outer surface of the ceramic insulator and a crimped portion located on a rear side of the tool engagement portion and crimped onto the shoulder portion of the ceramic insulator, an inner circumferential surface of the crimped portion having a region held in direct contact with the shoulder portion with a radially innermost point of the crimped portion being spaced radially apart from the ceramic insulator and axially apart from the shoulder portion, wherein the crimped portion of the metal shell and the shoulder portion of the ceramic insulator satisfy the relationships of 0.5≦(§A+§B)/§C and 0.25≦§A/§C≦0.6 where, when viewed in cross section through an axis of the spark plug and the radially innermost point of the crimped portion, §A is a radial distance from an outer generatrix line of the middle portion of the ceramic insulator to a first imaginary line extending through a radially innermost point of said region in contact with the shoulder portion in parallel with the spark plug axis; §B is a radial distance from the first imaginary line to a second imaginary line extending through the radially innermost point of the crimped portion in parallel with the spark plug axis: and §C is a difference between outer radii of the middle and rear portions of the ceramic insulator.