Patent ID: 7057332

Claim:
A spark plug comprising an electrically-insulating sleeve extending along a central axis of the plug, a first electrode mounted within the sleeve and having a tip projecting axially beyond an end portion of said sleeve, an electrically-conducting shell surrounding said sleeve, and a second electrode mounted on and electrically-connected to said shell, the second electrode having a tip positioned so that with the tip of said first electrode it defines a spark gap of the plug, the shell having an end portion terminating at an end surface of the shell, the sleeve extending past the end portion of the shell with a clearance therebetween and extending beyond said end surface of the shell, wherein the plug also comprises at least one projection formed from insulating material, the projection being positioned on the sleeve and extending circumferentially of the sleeve, the projection having a shape, in cross-section axially of the plug, such that a crest of the projection is located in the region of the end surface of the shell and reduces said clearance to lie in the range 0.5 to 1.1 mm, thereby forming a secondary spark gap.