A thermostat assembly including a flow blocking sleeve and a seat between which is defined a flow passage through which a fluid can flow and wherein the sleeve has an end opposing a bearing surface of the seat which extends perpendicular to an axis of the sleeve, and wherein the sleeve and the seat are movable relative to one another by movement of a piston of a thermostatic member to thereby control flow through the flow passage which is radial to the axis and wherein a gasket is crimped within a housing formed between a lateral wall adjacent the end of the sleeve and a ring so as to be secured to be normally engaged with the bearing surface to prevent flow through the flow passage unless the passage is opened by separation of the gasket from the bearing surface by movement of the piston in response to the thermostatic member being activated.