>I thought the bell-housing/transmission case was made from aluminum and 
just
>the upright engine cases were made from magnesium.  ??

An old fellow I knew near Denver, who rebuilt VW engines in his living
room (?), once told me a tale about grinding out the inside of a 6V
tranny bellhousing to acommodate a 12V flywheel. Had no problems with
fires etc while grinding... but then dropped a lit cigar end on the floor,
not noticing it had landed on some of the shavings, and stepped on it
with his boot. Got a VERY hot foot very quickly, he claims.

Seems a little hard to believe that a cigar would light what the grinder
didn't, but he was quite a storyteller as well as a pretty good mechanic.
His point was, the tranny housing is also a magnesium alloy. Maybe
identical to the engine crankcase-- anyone know?