I don't have the exact measurement handy, but on the 3L I just build, the pistons extended above the deck by a few thousands. As Ski says, the head gasket gives you some extra head room (if you will). As long as you have 35-40 thousandths or so (between top of piston and surface of head), you should be ok. Stock HG compressed thickness is anywhere from 1.25 to 1.5mm from what I've seen (if memoy serves), so that gives you some room for the pistons to extend above the head a tad. Measure twice, cut once.