What is core shift?

It is acutally present in most cast products. Heads, intakes, blocks, etc. The sand in a casting mold is the open areas in the finished part. So as an example, the cylinder bores have a casting core, the cooling jackets have a casting core... A block casting mold can contain a bunch of cores, and these sometimes shift while the process is being done. So you end up with the finished castings having rough parting lines, or thin spots as noted.