Bore vs piston
 

Another thread indicates that the block bore size is adjusted to the piston size according to the clearance required. I.E. for a 4.150 bore, the piston would be exactly 4.150, and the block bore would be adjusted according to the clearance required by the piston manufacturer. So, for a clearance required of .005, the bore would have to be 4.155? Would there be any chance of buying pistons to match a bore?

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.

Typically you buy pistons and bore/hone the block to get the proper piston to wall clearance. Are the bores you are referring to a fresh bore and hone?

Not harsh in any way, Mike. Thanks for the response, I asked because I read in another thread that the pistons were manufactured to the exact bore size (4.180) and the block was machined beyond 4.180 to suit. I was just a bit perplexed about that. I have read many years ago where a block would be bored to less than the overbore size and then honed to suit the piston which would be less than the overbore size.

Ag, yes. It was another thread/post where the pistons are at 4.180 and the block final bore is 4.184 or so. Darned if I can find that post again. That was the reason for the query.

Most off the shelf pistons have the clearance cast or forged in; I.E. cast are typically .002 under and forged are typically .004 under finished overbore.

Custom pistons can be ordered for any size you desire