Abstract: In order to attach a radiator cap 1 by means of a bayonet lock 4, 5 at a radiator neck 2 and to be able herein to install a completely assembled pressure relief valve 35 and vacuum relief valve 40 at the radiator cap 2, a lifting counterpart curved surface 23 is provided so as to be spaced axially from each bayonet engaging curved surface 5. In addition a counterpart cam 26 is at least indirectly connected with each bayonet cam 4, especially if it is manufactured in one piece, which counterpart cam in connection with the lifting counterpart curved surface 23 causes the necessary lifting of the radiator cap 1 counter to the installation direction when the radiator cap 1 is opened. Furthermore, the lifting counterpart curved surface 23 is expediently fastened at plastic part, according to one embodiment of the invention, constituting the stub or neck of a compensation vessel or container 45. This stub is preferably fabricated to form one piece with the vessel 45 .