This is what you get when you take an air cooled engine and add cast-on
water jackets to make it water cooled.   It was a kludge right from the
beginning, since a combustion pressure-tight compressed metal to metal seal
could only be attained between the cylinder sleeves and heads (just like the
air cooled engine), necessitating the rubber seal between the water jacket
and head.

However, it is a testament to good metallurgy and precision German assembly
that the original engines last as long as they do, but now aftermarket
replacement parts are not of the same quality.

For comparison, the Subaru case halves are cast in one piece with water
passages and bored cylinders like normal engines, and use a conventional
head gasket.  It, of course, can leak too.

One theory to reduce the electrolytic corrosion of the heads is to add an 8
guage ground wire from each head to the case.  The thought being that the
electrical contact between the heads and case is poor because of the stud
nut sealant and rubber gasket.  The main connection is through the cylinder
sleeve,  which has a crush gasket, and perhaps charge imbalances can build
up.

Who knows, but it's easy to do.

Good luck!

Stuart
'85 Westy--my fourth, and last, waterboxer engine