i believe core shift is when a new block, through repeated heating and cooling cycles,
'settles' in to its final state. this may cause the cylinder bores to distort in relation to the cooling passages, creating
offset and/or thinner walls. this usually occurs within the first 10k-20k miles. this distortion might be responsible for odd clearances between the cylinders and pistons. it also causes some cylinders to run hotter than others.

this is why many engine builders like to
use 'seasoned' blocks.

does anyone know more about this? and how it could be related to piston slap

 

Core shift refers to the casting process of the block. A" core" is the sand mold used in the casting. In some instances the core " shifts " slighty before or during the block pour resulting in thin or thick areas. You are correct in thinking blocks do change as they heat and cool repeatedly, but this is not core shift.