Your correct, aluminum expands eight times more than steel and cast iron when heated to the same temperatures as each other Google aluminum expansion rates, one of my old Crane Cam catalogs says to set the lash .002 loose when setting the valves on a cold cast iron motor, set them .006 tighter cold with aluminum heads on a cast iron block and set them .012 tighter cold on a aluminum block with aluminum heads. I found that information to be accurate on the first all aluminum street Hemi motor I built and on many other mtors since with iron blocks and aluminum heads you do want to check each motor to make sure those settings is what that motor needs and wants