I wouldn't run the piston closer than .035. In some class racing engines I have built in the past you get the piston to "witness" the head even at .035. No two engine parts are exactly the same so if you're getting things that close you need to treat the engine as if each cylinder were a separate engine. As far as piston to valve, you need a margin for error at some point in the engines life. I wouldn't run closer than .060 intake and .080 exhaust... Most builders like to see over .100 because of driver error. Unless it's a auto car and your keep a close eye on it race to race I wouldn't build it that close to crashing itself.