I think it has less to do with the heads being made of impure aluminum, but rather it's just that they are made of aluminum in the first place. Any aluminum head is going to suffer serious consequences if overheated, but thanks for the heads-up (pun intended) about not shaving too much off.

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1998 BMW 328is
1966 Pontiac GTO
2007 Subaru Impreza 2.5i 5-door
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