Aluminum heads crack or warp when overheated. Iron heads can survive overheating.
BMW-ATI, it sounds like you want to learn about cars. It's pretty obvious that you have little experience with cars, but quite bit of drive to gain some. I learned my stuff when I was about 12. My neighbor, who taught highschool auto class, gave me a textbook. Go to the local library and look up some books. They will be in the 629 area, with all of the mechanical stuff. You might even see a service manual for your bimmer. Once you have an idea of what you are looking at things will make sense. I did an engine swap for a buddy when I was in highscool. Why I didn't take auto in highscool, I don't remember.
97 328iS, Manual, Stock