By "core" do you mean that its' still on a nasty engine that's been sitting outside for 15yrs?
Or, have they been checked and complete, sitting dry on a shelf?
By "core" I mean they were removed from a running engine and put in the corner of my heated shop. Most people modify the heads anyway and have them ported & polished and install bigger valves. So, they pretty much rebuilt the heads to be like new.