Yeah the thicker shims pry the pedestal apart with the combination of heat cycling, bolt clamping force and the constant stresses of the valvetrain. If you think about it the pedestals themselves are relatively fragile to begin with and ironically the weakest pedestals (the ones that are hollowed for the oil passage) are the ones that are right between the heavier intake valves.... even though they are a lot wider they're not really that much stronger at the cup when you put a thick shim on them. I remember racers in the early 70's were already starting to mill them down for blocks because they didn't want to risk a ton of hours in ported iron heads and expensive valvetrain for a cast pedestal. There was a reason the Race W2's had blocks.