This X 2. Same goes for the cold weather 400 block nonsense. I’ve sonic checks those with results from excellent (.200+) to totally useless and tons of core shift.I still don't go by any of that info. I sonic check all of my blocks. I have seen so called thick wall blocks that can't even go +.030 over.
I have noticed variations in hardness but nothing that followed a specific year or casting number pattern and the factory blocks were all noticeably softer than the aftermarket blocks. My hone was an HP6A Diamond hone so it didn’t really care how hard the blocks were. The ductile iron sleeves I frequently used took a touch longer. I’d hate to hone those with a vitrified stone on a sunnen machine.What they don't talk about is the "hardness" of the later blocks.
I don't know what you guys have experienced, but I've noticed when drilling/grinding/etc on the later 70s blocks it's noticeably easier than the earlier ones, leading me to the conclusion that the later castings may be fine on thickness but softer in composition.