Disclosed is an improved spark plug having an electrode extending from an insulator tip into a piston chamber in use. The improvement comprises a shell defining: a cavity; a cavity opening which couples the cavity with said piston chamber in use; a conduit extending from the cavity and into which the electrode extends in spaced relation to the shell; and, with the tip, a chamber surrounding the tip in spaced relation and coupled to the cavity by the conduit. The cavity includes: a concave portion presenting towards the opening such that the cavity narrows as it extends towards the conduit; and a cylindrical portion extending from the concave portion towards the opening. The chamber includes a cylindrical portion surrounding the tip in spaced relation and a frustoconical portion coupling the cylindrical portion to the conduit such that the chamber narrows as it extends from its cylindrical portion to the conduit.