US Patent 10431961 Spark plug

A spark plug includes: an insulator having an axial hole formed therein in an axial direction; a center electrode extending in the axial direction and having a rear end located within the axial hole; a metal terminal extending in the axial direction and having a front end located rearward of the rear end of the center electrode within the axial hole; a resistor arranged between the center electrode and the metal terminal within the axial hole; and a conductive seal layer that fills a space between the resistor and the center electrode in the axial. The conductive seal layer has a first layer portion located adjacent to the center electrode and a second layer portion located between the first layer portion and the resistor. The second layer portion has a thermal expansion coefficient different from and falling between those of the first layer portion and the resistor.