With the change in casting technologies and engine construction techniques have come new, more reliable engines, and new challenges for the machinist.
For most, this has meant a reduction in the amount of bottom end work they are seeing, and a significant swing to cylinder head work. Of course, some problems that can be encountered aren’t necessarily new, but this doesn’t make them any less annoying.
Take the following situation for example: You have just finished putting together a beautiful set of heads for a customer’s big block Ford. You have meticulously checked and rechecked the heads for flatness, doing the appropriate milling and being very careful to provide the correct surface finish.
You fire off the heads to the customer and then the phone rings. Expecting a call thanking you for doing such a beautiful job, you’re surprised to have a miffed customer on the line, wondering what the heck you did to his heads because now the intake manifold won’t fit.
Since this was just a head job, you never saw the manifold, though you should have asked for it too. Now that you’ve done the heads, how are you going to get them to mate even if you do get hold of the manifold and the heads again, since the block might be sitting in the customer’s car?
There is actually a formula for figuring this out, if you know how much you have removed from the cylinder head surface, that is.
Assuming that you’re not a complete idiot and actually recorded this information, it should be no problem.
After machining the deck surface of the cylinder head, it is necessary to machine the intake manifold side of the head.
If more than .010″ of the surface of the head has been machined away, the boltholes in the intake manifold and head may not line up when reassembled.
On engines with the more traditional intake manifold construction, as opposed to those where the intake manifold also serves as the lifter valley cover, the manifold mating surface of the head has to be machined to correct the mismatch.
The amount to be removed varies with the amount removed from the head, of course, but also varies with the angle at which the head is oriented. For example, a head that has a head gasket and the intake manifold surface at a right angle (90 degrees) to each other would be considered a 0-degree head.
To ensure that the boltholes will line up, that mating surface must be machined.
Take a small-block Chevy for example. Assume 0.015″ was removed from the gasket surface of the heads of the 90 degree V8. Since the manifold mating surface of this engine is oriented so that it is regarded as a 10-degree head, the calculation is as follows: 0.015″ x 1.2 = .0180″, the solution being the amount to be removed on the intake side manifold mating surface of both heads.
The situation is a bit more complicated on modern V8 engines, where the intake manifold also acts as the lifter valley cover. This is as a result of the third mating surface for the intake manifold: the block.
Put simply, if the cylinder heads have been machined, the intake manifold may actually contact the block before it can mate with the intake surface of the heads. Think of it as being high centred. To correct the situation, material needs to be removed from either the block or the manifold, but as before it must be precisely the right amount.
To determine what this amount is, let’s go back to our small-block Chevy example.
If 0.015″ was removed from the cylinder head gasket surface, then: 0.015″ x 1.71 = 0.02565″, which is the amount to be removed from the block or manifold.
By paying attention to the potential effects of machining on the mating of other components, and making the adjustments as part of the process, you can avoid embarrassing situations and rework.