Sorry about that, I didn't word it correctly. When I mentioned a less pure aluminum, I was intending to state that as the reason why the heads crack so soon. Aluminum is aluminum and it will crack, but the quality of aluminum is responsible for the heads cracking without much warning. Usually noticing the temp guage in time when it reaches boiling will give a SMALL window to save the head, but with this case, the temp guage is already too late of a warning.
I guess, in short, I meant to say the quality aluminum is less resilient against the heat.
Sorry about the confusion.