Core shift? Explain plz

I keep reading about "selecting a good core" and avoiding core shift, when they say "core" do they JUST mean a bare block? I understand sorta about if the "core" or mold the block was cast in "shifts" the bores will not be equal, anyways....would someone care to explain everything? thnx

Core shift deals mainly with the cylinder wall thickness. When the block is cast, the core can move slightly. This can cause the cylinder wall thickness to be different around the cylinders. When the block is bored out to get a maximum displacement, you don't want walls thinner on one side of the cylinder than the other.

To check for core shift, the block needs to be sonic tested.

There's usually no concern about core shift unless you're trying to get the maximum displacement from the block. A 350 block bored +.080" or a 400 block bored +060". Only then do you need to make sure the cylinder walls will be thick enough for such an overbore.

Well my block is .030, so I guess I'm good....I did notice that 2 of my cylinders have slight oxidation patches.....not really rust where metal is chipping off, but slightly discolored patches of brown, will honing clean that up? Because to touch you can feel those patches are rougher....will another .010 rebore be needed to get a desired surface?