Spark plug assembly for internal combustion engine

An internal combustion engine (13) includes a cylinder head (12) and a spark plug assembly (10) adapted to be received in a cylinder head bore (20). The spark plug assembly (10) includes a metal ground electrode element (40) having an annulus (44) disposed on a transverse annular seat (30) of the cylinder head bore (20) and a ground side electrode (50) extending from the annulus (44). The ground side electrode (50) is received in an axial slot (32) of the cylinder head bore (20) between the annular seat (30) and a cylinder head inner wall (16) and protrudes into the combustion chamber (18). The orientation of the ground electrode (50) relative to the cylinder head (12), and thus to the geometry of the combustion chamber (18), is established by the circumferential location of the axial slot (32) relative to the bore (20). The spark plug assembly (10) also includes a centre electrode element (42) having a centre electrode (60) and surrounding insulator body (62). The insulator body (62) includes a shoulder (76) that is clamped against the ground electrode annulus (44) on the seat by a spanner retainer nut (82) threaded into the cylinder head bore (20) about the insulator body (62). The ground electrode element (40) and the centre electrode element (42) are thereby secured in position in the cylinder head bore (20).