E30 325 M20 CYLINDER HEAD CRACKING PROBLEMS

I'M A MOTORSPORT STUDENT AND AM BASING MY MAJOR PROJECT ON E30 325 M20 CYLINDER HEAD CRACKING PROBLEMS. THE PLAN IS TO POLISH AND PORT A HEAD, BUT I NEED TO DISCOVER THE MAJOR
REASONS OF WHY THEY GO WRONG IN THE FIRST PLACE. ANYTHING YOU FEEL MIGHT HELP WILL BE EXTREMELY APPRECIATE. THANKS ALOT
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OK OK, dear student, Like most of others, you'll arrive in a new company whith your degree and try to rebuild the world, shouting everywhere, pumping the air in front of you, putting it behind. You're a ventilator dear student !
Don't forget that you're invited in the newsgroups, you're not at home. So, please behave accordingly. It would be better you use low-case characters unless your caps lock key is stuck On.
Please write again your mesage in a friendly manner.
Hope you'll understand. TIA.

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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.

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