Patent ID: 8058786

Claim:
A small diameter spark plug comprising: a metal shell having a main body portion including a first end, a second end and an intermediate section that collectively define an interior cavity, the intermediate section including a first unthreaded zone defining a first diametric portion and a second unthreaded zone adjacent to the first unthreaded zone defining a second diametric portion, the first diametric portion having an inner periphery greater than an inner periphery of the second diametric portion, the metal shell further comprising a threaded zone adjacent to the second unthreaded zone and a fourth unthreaded zone adjacent the threaded zone, the threaded zone defining a third diametric portion having an inner periphery less than the inner periphery of the second diametric portion and the fourth unthreaded zone defining a fourth diametric portion having an inner periphery, the inner periphery of the fourth diametric portion being less than the inner periphery of the third diametric portion, wherein the interior cavity has a stepped configuration defined by the inner periphery of the first, second, third and fourth diametric portions; a ground electrode extending from the fourth diametric portion; an insulator disposed in the metal shell, the insulator having a first end portion extending from the first end portion of the metal shell, a second end portion provided at the second end of the metal shell and an intermediate portion being configured to be recessed within the stepped configuration of the interior cavity and the intermediate portion extending between the first end portion and second end portion, the intermediate portion having a first diametric section nested within the first diametric portion and a second diametric section nested within the second diametric portion and a third diametric section nested with the third diametric portion, the first diametric section having a material thickness greater than a material thickness of the second diametric section; a center wire disposed within a central bore extending through the insulator from the first end portion to the second end portion of the insulator, the center wire having a head portion positioned within the second diametric section of the insulator; and a resistor disposed in the central bore of the insulator, the resistor being in electrical communication with the center wire via the head portion and the resistor is located within the first diametric section and the second diametric section of the insulator, wherein a minimum thickness of at least 2.0 mm of the insulator is located between the resistor and the first diametric portion and the second diametric portion of the intermediate section of the metal shell and wherein an outer diameter of the threaded zone is no greater than 12 mm.