Not trying to get in to an argument, nevertheless, I can't see how can Hard Blok have the same thermal expansion as gray cast iron.
The expansion rates of aggregates and gray cast iron are very different. Also, Hard Blok does not contain cast iron in its composition.
Although dangerous to use, Methylene Chloride will dissolve Hard Blok.
Below is another past discussion on the subject:
http://classracer.com/classforum/showthread.php?t=14045