A totally inappropriate response to a valid question.
Perhaps you should try kicking your dog a few time before you post again.

On the original question.
I am aware of two problems that led to cracked heads on the e30 neither of
which actually had anything to do with the head other than the fact that
they are aluminum alloy which is especially vulnerable to overheating.
1)  The heads are very tall and so put extra stress on the headbolts due to
thermal expansion differences between the head and the bolts.
This led to some head bolt failures early in the production run of the M20
engine.  The head bolt failures sometimes resulted in overheating and
cracking the head.  The fix was to incorporate 'softer' head bolts with a
reduced diameter shank.  This reduced the stress in the threads to achieve
longer fatigue life.
2)  At some point in production water pumps utilizing plastic impellors
molded onto metal shafts were incorporated as a cost saving measure.  The
impellors contained high residual shrinkage stresses from cooling around the
shaft.  Some of the impellors cracked with subsequent loss of function and
overheating of the engine.  I am not sure if this item was relevant to the
e30 or only to later models.  Perhaps someone else has better information.

You can also google e30 and/or M20 engine as there is a lot of information
available on the internet.