US Patent 10014665 Spark plug

A spark plug having an insulator that includes a first portion which accommodates a front end of the metallic terminal. A portion of the metallic terminal has a roughened surface portion with a Vickers hardness of 200 Hv or greater and 320 Hv or smaller. A first ratio of an outside diameter of the roughened surface corresponding to at least a part of the first portion of the insulator to the first bore diameter of the insulator is 0.90 or greater. A second ratio of the first bore diameter to the second bore diameter of the insulator is 0.80 or greater and 0.98 or smaller.