A method of manufacturing a spark plug (20) for being threaded into a cylinder head (28) of an internal combustion engine is provided. The spark plug (20) includes a shell (24) with threads (26) disposed at a predetermined rotational position (a) relative to a shell outer surface (64) and ground electrode (34). The position of the threads (26) relative to the ground electrode (34) places the ground electrode (34) in a desired position in the combustion chamber (22) and relative to components of the engine, thus allowing the ground electrode (34) to provide a robust and reliable ignition. When multiple spark plugs (20) are formed, the threads (26) in each of the shells (24) are repeatedly and accurately formed at the predetermined rotational position (a) by locating the ground electrode (34), threads (26), and dies (76) of a thread forming apparatus (102) in specific locations.

A spark plug includes an insulator having an axial hole formed therethrough in the direction of an axis of the spark plug, a center electrode disposed in a front side of the axial hole and a metal shell disposed around an outer circumference of the insulator and having a collar-shaped tool engagement portion and a crimp portion located on a rear side of the tool engagement portion and reduced in diameter toward the rear, wherein the crimp portion of the metal shell is made of a carbon steel material having a grain size number equal to or greater than No. 11 as measured according to JIS G 0551.

A spark plug includes an insulator having an axial hole formed therethrough in the direction of an axis of the spark plug, a center electrode disposed in a front side of the axial hole and a metal shell disposed around an outer circumference of the insulator and having a collar-shaped tool engagement portion and a crimp portion located on a rear side of the tool engagement portion and reduced in diameter toward the rear, wherein the crimp portion of the metal shell is made of a carbon steel material having a grain size number equal to or greater than No. 11 as measured according to JIS G 0551.