Abstract: A thermostatic cartridge assembly for a by-pass valve that regulates fluid flow between two different fluid circuits includes a casing defining a first fluid port and a second port; a thermostatic body having a piston movable along a central axis of the thermostatic body in response to heating and expansion of the thermally responsive material; a cap joined to a second end of the casing; a biasing member urging an end of the piston against a roof of the casing; and a sleeve carried by the thermostatic body, the sleeve having a circumferential side wall and a ceiling that seat against surfaces of the casing to block the first fluid port when the thermally responsive material is at a first temperature, the circumferential side wall blocking the second fluid port when the thermally responsive material is at a second temperature greater than the first temperature.