In another, secret, life, I'm an Alfisto. A good friend is the original owner
of a 1974 ALFA GTV (very similar to mine, it turns out), and has recently
discovered that using distilled or deionized water is not enough to avoid
electrolysis corrosion (the car has an aluminum block and head). The hard way.
He found that his coolant was disappearing -- not leaking, not boiling out,
just disappearing. Where? Well, there was this distinctive burbling sound when
he shut the engine off. He described it to a mechanic, who knew immediately
that it was common failure mode number 137: the head is thinnest around one of
the exhaust valves, electrolysis had eaten throught there, and coolant was
passing into the exhaust. The burbling was the sound of coolant boiling away.
He knew that this was a potential problem, and therefore NEVER put tap water
in. It wasn't enough.
There are sacrificial elements available that replace one of the freeze plugs
in the head. If you can find one for your aluminum head, do so...
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