Not to sound harsh, but you can buy pistons in any exact size you wish. You specify what size you want and the piston manufacturer makes them to your specs. Since this is in the street section though, I am guessing you are asking about what are typically called "shelf pistons" These pistons are manufactured slightly smaller than the nominal bore. Meaning essentially, the clearance is built -in to the nominal bore size. In your example, if you ordered a +.030 shelf cast piston for a 400 Pontiac, the piston at it's largest measuring point would be 4.148-4.1485 for the .0015-.002 clearance it would need. A forged piston would be a little smaller for the extra clearance required. Machine shops that do quality work, NEVER finish hone a block until they have the pistons in hand and have measured the exact pistons going in the block. Cast pistons are usually pretty close in size through a set, usually within .0005. Custom pistons have more variation in them because they are not mass produced thousands at a time. A good machine shop will hone the bore to the specific piston to wall clearance for each piston based on it's actual exact size. Hope this helps.
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