High thread spark plug with non-axisymmetric ground shield for precise ground strap orientation

Abstract

A spark plug is provided that ensures that a ground electrode is positioned in a predefined, precise orientation when installed in a spark plug hole of an engine head. The spark plug is configured for axial insertion into the plug hole, and has a non-axisymmetric ground shield that fits into the plug hole, wherein the ground shield has an outer shield surface with a non-axisymmetric shape and the plug hole is also provided with a complementary non-axisymmetric shape. The spark plug includes a central insulator, which has an inner end surrounding a central electrode and supporting the ground shield, which is mounted on the insulator to support a ground strap adjacent the electrode for forming a spark therebetween. The insulator and ground shield are axially slidable into the plug hole, and the spark plug includes a jamb nut which is rotatable to fix the spark plug in position in a predefined orientation. Preferably, an outer surface of the insulator and an inner surface of the ground shield have complementary shapes wherein the ground shield fits closely on the insulator. For example, the outer insulator surface and the inner shield surface may have a complementary axisymmetric shape, such as cylindrical, or a non-axisymmetric shape, which may conform to the non-axisymmetric shape of the outer shield surface.