Another argument can be made for the use of aluminum heads and it's called repair-ability. Cast iron does not weld very well. Break a piston and it'll quickly tear through a cylinder head. There are methods to repair a cast-iron head, but they're expensive and not easily done. And most times you'll end up with a crack down the line. Mitchell says, "So what; a cast head is typically very inexpensive, so throw it out and get a new one." This is fine unless you've got a ton invested in porting or valve work. Starting with a new head, you'll have to invest that money all over again. But Mitchell continues, "We sell a number of cast-iron heads that will make good power without any extra port work, so replacing one down the road because it's broken is not an expensive affair." Any heli-arc welder worth his weight, though, can usually bring a torched aluminum head back to life. A little bit of clean up and it'll be as good as new.
So what's better, aluminum or cast iron? That depends, and it'll have to be a decision for you to make. But don't be fooled into believing the myth, "Aluminum heads make more power."
Myth #2 -Bronze Valve GuidesWho ever thought of the idea that bronze valve guides are better than any others? Years ago, when we used to rummage through scrap yards for a good set of cylinder heads, typically, we'd come across a good casting, but the valve guides would be worn out. What we then did was bore out the guide and press in another one. While bronze guides were easily available to most cylinder head rebuilders, that's what was used. Basically, they were just a cheap rebuilder's trick. But are they better?
Let's first talk about aluminum heads. It's very important to use another material for valve guides in aluminum heads because a steel valve will wear out the aluminum, but in reality, cast iron has better wear characteristics than anything. Mitchell says, "The fact is, Chevrolet uses cast-iron guides in their aluminum Corvette heads, even in the exotic Z06 models. If it's okay for them, why do we want to re-engineer it?"
Cast-iron heads are another story. Whenever you bore out a valve guide, you weaken the casting in that area. Add in the fact that the interference fit between the guide and casting might not be right, coupled with the fact that the valve job must now be completely redone (because it's no longer concentric with the valve bowl), and you've created more problems than it's worth. Actually, bronze guides aren't worth much to begin with.
Fortunately, today we don't have to go scrounging junkyards to find a good set of castings. Mitchell's company (World Products) casts and sells a number of top-quality cast-iron (as well as aluminum) heads that make good power right out of the box. They also incorporate integral cast-iron guides on all their cast heads. Why would you want to take a brand-new casting and trash it by boring out the guides and installing bronze replacements?
Here's one other point to bring up on the subject of guides. Titanium valves, for those of you who use them, actually like the softer bronze material. It all really comes down to the interaction between two dissimilar metals. Don't be fooled by the myth, "Bronze valve guides are better.
Myth #3 - Splayed Four-Bolt Main Caps And Other MetalsAnother myth that goes back to our "junkyard days" is that four-bolt main caps that have the outer two bolts splayed (or angled outward) are better. When all we had to work with years ago were two-bolt blocks, a common practice was to install four-bolt main caps. This was a must for anyone making power, because the standard cast-iron two-bolt caps were not only weak (metal-wise) but caused web-splitting of the blocks. An easy fix was to install a four-bolt cap.