Re: Flathead Overheating

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Are you sure you had the cooling system full of coolant before you took off for the first 5 miles.  Needing a gallon of water on a fresh overhaul after 5 miles would indicate that.  Just filling the radiator with water may not fill the entire cooling system but letting the engine warm up until the thermostats open and being sure there is circulation will make sure.  The other issue, I looked up your previous post, is that your rebuilder has swapped the head gaskets side to side.  It would not be the first time that has happened (I speak from experience here).  Although the gaskets look the same they are not exactly mirrors of each other.  If you are not losing water on the ground then it has to be going either into the pan or out the exhaust neither of which is a good option.  Hope you find the problem soon.