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There was an article I saw on this subject for the 427 and 428 FE blocks either in an old copy of Shelby American or SuperFord.

They did some block research using sonic testing to cull out those blocks that on the surface appeared to be "overboreABLE". The sonic testing revealed that all cylinder walls are not alike, some were perfect and thicker while perhaps one of the eight was very thin causing the block to be considered unboreable.

The hard part was to find a block that had sufficient wall thicknes and concentricity to allow uniform overboring for ALL eight cylinders.

They were able to use sonic testing equipment to find such blocks.

The take away factoids I remembered were:

1) that not all blocks can be overbored due to one or more thin wall(s)
2) FE blocks with equal wall thickness all around are somewhat rare
3) overboring reveals occlusions or voids at times
4) old blocks are stable and pretty much stress relieved due to thermal cycling