A spark plug and a method of manufacturing the same, where the spark plug has a metal shell, an insulator, a center electrode, a ground electrode, and a gasket located over a threaded portion of the shell and used to seal the spark plug against a cylinder head. Once the gasket is attached on the spark plug, it is shrunk to become a post-formed gasket with an inner diameter that prevents the post-formed gasket from slipping over a threaded portion of the metallic shell. This process may be carried out with a collet type machine that produces a post-formed gasket that is flat and has a substantially uniform thickness, which can improve the sealability of the gasket. This may be important when the spark plug is installed in a cylinder head made from a lost foam casting process or other process that creates a somewhat porous sealing surface.