[Half-Inch Stud]

Will,

YES, .0015" - .002" in concentricity DOES seem insignificant to me, at the boring/honing stage, on a Pontiac block. Further to my point is the "perfect bore" achieved at 70F machineshop room temperature does not assure whatsoever any sort of concentricity at 180F average running temperature....

By virtue of having different temperatures throughout the block inner and outer case will distor the "Perfect Bore" to beyond .0015 - .002". I wager .003" is likely.

considering the bottom of the cylinders may be at 140F [bore cross hatch should be zero, polished], while the mid-range cylinder bore is sitting around 170F, while the 1" top-Cylinder bore region sits above 200F on the inside walls.

The pistons are not that good. I had my pistons matched to the bores, after the bore/hone. Machineshops perform the machine-hone to reach the selected piston. In other words; .030" over really means .028" to .032" range for the piston and block at the same 70F shop temperature.

Does anybody take a close look at the piston thermal paths around the pin? You see, the pin to rod is a significant thermal path, along with the rings to wall. So, the pistons are not round [Top-view], nor are they cylindrical [Pin Side-View], Yet they may be substantially cylindrical [Skirt side-View] but there is no sealing wear-force on the non-skirt sides.

So, for good sealing, we're talking about the RINGS seeing a good support system by the ring lands. I doubt the rings are as round as some folks assume, for the set-gap.

Then, I truely doubt the ring sealing force imposed upon the upper cylinder walls is uniform, particularly knowing the rings are stiff "C" clips under thermal stress.

SOLUTION:
Let the rings define ROUND at running temperature.
Obey your instincts and instructions during engine ring break-in period. ROUND will be difined best at running temperature. Cross-hatch will survive in the regions where it is most important.

NOTE:
I am surprized at how quiet this [basic, yet important] topic has been.

H.I. Stud