Keith Black designed the piston to be run in a tight bore. For street use you want .002-.0025". This is the difference between the piston diameter and the bore diameter.
You should be able to stick a piston in the bore and slide a .002 feeler gauge between it and the bore on one side only. When the piston is centered in the bore there should be .001 clearance on each side. If you mike the bore and the piston and subtract the two numbers you should get .002 and they recommend no more than .0025. I put my Egge cast pistons in at .003 with one hole @ .0035
tmm