while a lot of places refer to it as hot tanking, they don't actually hot tank anymore due to epa regs, but place the block in a burn off oven instead and burn everything off. That can still leave ash and other scale type deposits in it.

That could explain it. I'll get it clarified when I take the Z's engine in. I'm going to call the local Z club before I do it and see who the members usually use when it's time.