I have no idea where this wives tale came from. Aluminum is less prone to cracking and warping than cast iron is. It cools down faster, throws heat faster, and is repairable, where cast iron has none of these features.
Almost every car produced these days has aluminum heads, and some engines are completely aluminum, and how many do you hear about having cracking or warping problems? None. Cast iron on the other hand is very prone to cracking, especially in high heat conditions. I’ll show you a whole pile of junk, cracked cast iron heads out back of my shop, and there isn’t a single aluminum head in that pile, and we work with more aluminum heads than cast iron (by like 10 to 1).
I believe where aluminum heads got part of their bad reputation was back in the 70's when the Chevy Vega and other smaller American cars were first beginning to get aluminum heads and were blowing head gaskets on a pretty regular basis after a given amount of miles. It wasn't the aluminum's fault though, it was the machining process done at the factory that was the problem. When you machine the deck surfaces of cast iron blocks and heads you usually set the feed speed of the machine so that it makes little ridges in the surface to "grab" the head gasket and lock it in place. The problem with this back then was; no one considered that aluminum expands twice as much as cast iron, so what was happening was the rough machined surface of the aluminum head was acting like a mini draw file or rasp, skating back and forth across the gasket which literally tore it up causing it to eventually let go. The heads kept getting blamed for "blowing head gaskets", when in fact all it was was the machining finish on the head surface that needed to be smoothed out. Now when you look at the deck surfaces of aluminum heads they are super smooth, and almost shiny because they are so smooth, so when the head expands, that surface smoothly skates over the gasket without damaging it. You can see how shiny and smooth the surface of the head is in the picture to the right.
If you'd like to learn a LOT more about heads and the entire valve train, we have an awesome video on it called "Heads, Porting & Valve Trains" that you'd probably really like. It's a multi disk set with over 5 hours of easy to understand info in it that covers the entire valve train. You can check out that title and many others on our Auto Shop Videos page by clicking HERE.
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