The radiator and gas tank caps look alike between the years 1928-29 and 1930-31 respectively. But - they are different internally. The gas cap IS vented, while the radiator cap is not, as others have posted. It wouldn't hurt anything to run a vented gas cap on the radiator (although you might get some "weeping" coolant droplets from underneath), but you cannot run a radiator cap on the gas tank. Because the fuel system in a Model A relies upon gravity, it must a source of fresh air intake somewhere to allow nature's gravity to make the gasoline flow to the carburetor. If that air intake source is closed off (via an unvented or clogged vented cap), insufficient fuel will reach the carb and the engine will act like it is running out of gas - which it is. There will be enough of a fuel trickle to keep the engine idling, but will peter out as power demands are required.
Marshall
