[w-lewis]

My original expansion tank was a beautiful, custom made "427 look alike" tank, made for my 351W. It utilized an original Ford thermostat housing that used factory gaskets. The tank was somewhat fragile as you could not press down on coolant cap without supporting underside of the tank with your other hand for fear the neck to tank weld could break. I removed tank several times and replaced gasket, but leak always returned. Odd thing is, it always came back after a week or two, usually while car sat up. Assumming the leak originated from a pinhole or the tank neck's weld, I replaced it with a sturdy welded aluminum tank from Finishline that utilizes O rings between the thermostat housing and mounting bracket and a conventional Ford gasket between the bracket and manifold. The new tank did not leak untill several weeks after installation, the last few weeks while the car sat idle. I've not put dye in the coolant, hoping to give me some clue as to where it's coming from.