There were a handful of revisions in the HG until mid production run of the 87 MY (and a few after that as well). I would wager that the majority of people who assumed they had a cracked head (and didnt test it) really had a hg failure between #1 and #2.

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http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42