Patent ID: 8051822

Claim:
A spark plug and cylinder head assembly comprising: a cylinder head of an engine having formed therein a bore, the cylinder head also having a surface which faces a combustion chamber of the engine and on which the bore opens; and a spark plug for igniting an air/fuel mixture in the combustion chamber of the engine, the spark plug including a) a tubular metal shell fit in the bore of the cylinder head with a longitudinal direction of the metal shell perpendicular to the surface of the cylinder head, the metal shell having a distalmost end surface facing the combustion chamber and an inner shoulder that is formed on an inner periphery of the metal shell spaced away from the distalmost end surface in the longitudinal direction, b) an insulator having an outer shoulder formed on an outer periphery of the insulator, the insulator being retained in the metal shell through an engagement between the inner shoulder of the metal shell and the outer shoulder of the insulator, c) an air pocket formed between the metal shell and the insulator, the air pocket extending, in the longitudinal direction of the metal shell, from the inner shoulder to the end surface of the metal shell to open to the combustion chamber, d) a center electrode secured in the insulator, and e) a ground electrode joined to the metal shell and facing the center electrode through a spark gap formed therebetween, wherein the distalmost end surface of the metal shell has a radially outer edge and a radially inner edge, and tapers from the radially outer edge to the radially inner edge in a direction toward the inner shoulder of the metal shell, whereby the end surface tapers from the distalmost and radially outermost edge thereof to the radially inner edge, a groove is formed between the metal shell and the cylinder head around the outer edge of the end surface of the metal shell, and the groove has a depth in the longitudinal direction of the metal shell in a range of 0.5 to 1.5 mm.