At 08:23 AM 4/20/2004 -0400, you wrote:

Tommy/John,
 
    Why are you guys using "tap water"...?  Shouldn't you be using distilled water in an aluminum block and aluminum heat exchanger...?

        There are several issues. Typically, tap water will work just fine. If you have very hard water in your area, you should probably use distilled water, or water that has been run through a water softener.

>>> Dissolved solids <<

        The main difference between tap water and distilled water (for use in a cooling system) is dissolved solids. These  are minerals like salt and calcium that are dissolved in the water. These minerals can come out of solution and attach to the surfaces within the cooling system. Ordinarily, this is not a problem in a closed-loop cooling system unless the tap water is very hard.

>>> Corrosion <<<

        Corrosion is another, more serious, issue. Distilled water won't help you much (and may very well hurt you.) The oxygen dissolved in the water along the dissimilar metals present in the engine are primarily what causes corrosion. The addition of rust inhibitor (present in antifreeze) protects the system from corrosion. The rust inhibitor raises the pH of the coolant so that the metal does not want to go into solution (dissolve) in the coolant. It also helps keep the oxygen from attacking the metal.

>>> Galvanic action <<
        The radiator is very often a different metal than the engine block (or other major components wetted by the coolant.) Because of galvanic action, it is very important that the radiator be electrically isolated from the chassis. Otherwise, the two metals will form a battery and current will flow, plating one component with metal from the other.

        You will notice that the radiator in your car is mounted using rubber grommets or plastic parts. It is electrically isolated from the frame.  Often, folks will discard the rubber grommets when they R&R the radiator. This happens a lot in body shops. If you electrically connect the radiator to the frame in your car, it will spring a leak in about a year.

        It is wise to rubber mount the radiator in your airplane.

>>> Closed sytem <<<
        One of the advantages of having a truly closed system, with a purge tank large enough to hold the expanding coolant, is that the coolant is never exposed to the air. In the systems with a small purge tank and a large overflow tank, the hot coolant is exposed to the air and absorbs oxygen. It is then drawn back into the cooling system where the oxygen can corrode the engine parts.