Abstract: A down flow, two pass radiator includes a means for selectively venting air displaced during coolant fill from one side of the top tank flow baffle to the other, while the baffle remains fluid tight during normal operation. The uppermost wall of the top tank slopes continually upwardly from the far, low end of the tank to a filler neck at the opposite, high end. A vent inlet through the baffle, near the inner surface of the uppermost wall, may be covered and uncovered by a blocking valve accessible from outside the top tank. The valve is left closed during normal radiator operation, but opened during coolant fill to allow free escape of displaced air across the baffle and out through the open filler neck.