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If you google "galvanic series", you'll see that brass is a more "noble" metal than aluminium alloy. This means that the aluminium alloy will corrode when in contact with brass, but because of the size of the radiator (assuming it's aluminium alloy?) in relation to the size of the bolt, the amount of corrosion that occurs on the radiator will be spread over a wide area and wouldn't cause a problem.
However, you don't want a bolt rolling around in the bottom of the radiator and possibly getting into the engine.
Cheers,
Glen
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