at the inner circumferential surface of the boring 4 therefore installation of the spark plug 2 is accomplished by merely plugging in with the spark plug 2 being brought into retaining engagement with the cylinder head 6 with the aid of the retainer ring 24 for removing the spark plug 2 the retainer ring 24 must be brought out of engagement with the groove 26 in the boring 4 therefore the retainer ring 24 in the embodiment represented is constructed as a circlip in order to be able to release the retaining engagement with the cylinder head 6 preferably the retainer ring 24 that is structured as a circlip can be provided at both of its ends with raised lugs conical pins or a clip in order to be able to more easily accomplish the reduction of the diameter of the retainer ring 24 for releasing the spark plug from the boring 4 as can be recognized from the accompanying figure in the installed condition of the spark plug 2 its plug body 8 lies with its outer circumferential surface essentially flush against the inner circumferential surface of the boring 4 realized by this is a particularly good heat transfer from the spark plug 2 to the cylinder head 6 in order to be able to improve the heat transfer still further heat conducting pastes can be applied on the circumferential surface of the plug body 8 constructed adjacent to the lower end 2b of the spark plug 2 at the outer circumferential surface of the plug body 8 is another circular groove 28 in which is.