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Is that the well for the cold weather heater used to warm up the radiator fluid (and extended to warming up the cooling system for 'cold starts')? Just curious. If you lost a cap, maybe try a rubber cap that comes with various sizes of pipe or electrical conduit. If not, wrap it with some rubberized 'stretchy' tape that adheres to itself making one 'glob' of rubber (if it is of concern, that is). Good luck.


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