Piston protusion

[Matty]

I could be way off here but are you sure the block surface is completely flat?

I'd think you'd probably notice if the surface wasn't flat. As if Piston 1, 2 & 3 are at one level, then 4 at a different. That would mean the surface is fine until number 4. I don't think a drop of 1mm is going to be likely, and would be fairly noticeable I would of thought! Might be wrong though...