US Patent 11050221 Spark plug with anti-loosening feature

A spark plug includes a tubular metallic shell having an attachment screw portion having an external thread, an insulator disposed inside the metallic shell and having an axial hole, and a center electrode disposed in the axial hole. The attachment screw portion has a pitch diameter local maximum portion at which the external thread has a locally maximum pitch diameter. The pitch diameter local maximum portion is located on the forward end side of a rear end of the center electrode in a direction along an axial line of the metallic shell.