I think its because when he shaved the head it moves it closer too the pistons= more compression, but when u start adding big lift cams it pushes the valves further and they will hit the pistions.
We have been doing this around here with alot of luck building motors for race cars and our rock crawlers, but we have too grind down the top of the pistion some for better valve clearence becuase we are shaveing so much off but it makes them run strong with lots of power.
