It occurs all over the place in our Morgans. The pic below shows my engine block rotting away owing to the galvanic action between the Ali block and Cu/brass radiator (the rot is very slow though).
Sorry for being thick but surely the block isn't in contact with the rad? Or is it the coolant that does it? In which case isn't the water in the coolant that causes it? (maybe evans waterless stuff would be better)
The block in the pic looks fine to me. My 1962 Singer gazelle has an alloy thermostat housing bolted onto a cast iron block and its over 50 years old and neither has rotted away or needed replacement...yet - but you did say it was slow! ( like the gazelle!!)
