Abstract: A thermostat top part is mounted to a valve housing for rotation relative thereto and is axially removably secured thereto. The thermostat top part has a retaining ring surrounding and rotatable relative to a hollow cylindrical base foot. A clamping element is mounted by the base foot for radial movement between a position abutting against a clamping face of the valve housing that is tapered axially toward the housing abutment face in a radial inward direction for retaining the top part on the valve housing and a radial outward position permitting the thermostat top part being axially removed from the valve housing. A torsion spring resiliently urges the retainer ring to rotate in a direction for moving the clamping element radially inwardly.