Are aluminum heads prone to cracking and warping?

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 pretty much has none of these features. 

Almost every car produced these days has aluminum heads, and some engines are completely aluminum. 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.

When my shop was open (before I retired after about 30 years) I could show you a whole pile of junk cast iron heads out back of my shop, and there wasn’t a single aluminum head in that pile. And by far, I worked with more aluminum heads than cast iron (by like 10 to 1) so if aluminum was bad, that pile wouldn't have been made up of cast iron heads, but rather, aluminum heads... but it wasn't.

In the USA, aluminum heads got their bad reputation back in the 70's when the Chevy Vega and other smaller American cars that were first equipped with aluminum heads 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 that "grab" the head gasket and lock it in place. Actually, let me back up here for a moment because most factory engines, like your typical small and big block Chevys and Fords, didn't actually have "milled" or "machined" deck surfaces and were instead "broached", which is a different process than a rotating head on a mill like we use in machine shops, which it left a different texture on the deck surfaces of the blocks than milling does. I just wanted to clarify that, which is your useless knowledge for the day :-) Heads however, were in fact "machined" which did leave a rather rough surface compared to the decks of blocks. 

The problem with this back then was; no one considered that aluminum expands twice as much as cast iron does, so what was happening was the rough machined surface of the aluminum head was acting like a mini draw file or cheese grater, skating back and forth across the soft composition head gasket which literally tore it up causing it to eventually fail and leak. The aluminum 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're super smooth and shiny so when the head expands, that surface smoothly slides over the gasket without damaging it. You can see how shiny and smooth the surface of the head is in this picture. This is why you don't see head gasket issues anymore with aluminum heads. Who would have thought? 

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