its a measurement thing, the chain is x" long and figured to fit around the crank and cam gears. By decking the head and or block you are reducing the distance from centerline to centerline of the crank shaft and cam....creating slack. Not enough slack that you can move to the next tooth and have the tensioner pickit up, but enough to retard the cam slightly. Lets say the head is cut .020"....well you just made the chain .040" too long. In the 22R-EB Gen 1 motor the retarded cam timing actually boosted its power and widened its RPM range, but that head eventually cracked out...stock dimensioned head did fine too. Hope that doesnt confuse you any more.

~Darin <img src="/forums/images/graemlins/baby.gif" alt="" />