Abstract: An internal combustion engine includes a cylinder head and a permanent ground electrode and replaceable center electrode element received in a cylinder head bore. The permanent ground electrode is affixed to the cylinder head at the inner wall thereof and protrudes into the combustion chamber. The ground electrode preferably has a cup-like configuration defining an electrode-receiving hollow in the combustion chamber in registry with the bore. The center electrode element comprises a center electrode and an insulator body having a shoulder for engaging a seat in the bore. The insulator body shoulder is clamped against the seat by an annular retainer nut. When the center electrode element is so clamped, the center electrode is received in the electrode-receiving hollow defined by the ground electrode in the combustion chamber so as to cooperate with the ground electrode in defining a spark gap therebetween.