Cylinder head repair accounts for more than half of the work we do in my Automotive Machine Shop business. It is probably one of the most common engine repairs done in most repair shops. We also do quite a bit of performance cylinder head work.
Modern engines with thier aluminum cylinder heads and overhead camshafts are very reliable but they have a couple of weaknesses compared to the older V-8 engines.
1. The aluminum heads are not very forgiving when it comes to overheating. They will warp and can crack pretty easily. That is one reason to stress to your friends, family and customers, that if at all possible do not run a modern engine hot.
2. The overhead camshafts, usually driven by a timing belt, are very efficient but if the timing belt breaks . . . well lets just say that when the valves stop moving but the pistons dont, something has to give and it is usually the valves.
So the most common cylinder head repairs we see on new cars are from overheating or leaking head gaskets. On these heads we will check them for straightness first, then check them for cracks, and then resurface them if possible.
With a broken timing belt we will have to disassemble the head, clean and check all of the valves and valve guides, replace all of the bent and broken parts and reassemble them.
On an overhead camshaft cylinder head that has suffered an overheat condition or a blown head gasket, there are several things that people dont realize when they bring the head to the machine shop for repair.
1. If the cylinder head is warped more than a small amount, usually in the area of .003" or more, it must be "straightened" before it can be resurfaced. The reason for this is that not only the gasket surface is warped, the entire head is. This includes the part that the camshaft turns in. Cylinder head milling will make the gasket surface straight again but that does nothing for the rest of the head.
But But But . . . the engine was running ok before we pulled the head off ??
Yes it was. To be honest with you, before the head is unbolted from the engine it is probably perfectly straight, the bolts are holding it that way. But when it was overheated, a bunch of stress formed and as soon as you loosen the bolts, the head will usually spring upwards in the middle.
2. Are you sure the head gasket was leaking ?? Click on this link to see some pictures of leaking head gaskets.