I've heard the term "core shift" a couple of times lately regarding engine blocks. I have never heard of that before. Can someone explain what it is and what causes it?

The way I understand it is when things like blocks or intake manifolds are cast, they are cast of molten metal poured into a mold. But something like an intake manifold with have two molds, one for the outside shape and one for the inside shape, then the metal poured in between. The inside mold would be the core. But if the two molds don't stay perfectly aligned through the whole process, you get what's called core shift, because the 'core' has shifted away from where it's supposed to be. I assume that happens because of the cooling process or just from the pouring of the metal.

Someone correct me if I'm wrong.

From what I understand, the block is cast and the internal core moves somewhat during casting.... The bore thickness is then different.

Here is a thread with some in depth information on this problem. I know, I know it's a GM site but the info is very good.