All (almost) the answers that everyone is looking for on the subject
of how to protect their wasserboxer cooling system, heads and etc. can
be found at:
http://www.speedquest.com/ara/TOC/Cams/contents/UpOnJacks/jacks1.htm
It seems that Alfa Romeo owners have the same concerns and one of
theirs offers his knowledge and wisdon on the above quoted page. Type
of water to use, ionic chemistry, sacrificial offerings, anodes, etc.
all the answers to all the messages seem to be there. The only thing
left out is where to get and where to put a sacrificial magnesium plug
for a wasserboxer. The sacrificial anode (magnesium plug) must be
electrically connected to the engine block in order to protect it (and
therefor make the block the cathode!).

John Denis  P.Eng.
Stebro Exhaust products
'97 Diesel Transporter
'89 Alfa Romeo 75
'74 Datsun 260Z