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Engine rebuild step two:

MACHINING A CADILLAC

OVERHEAD-VALVE V-8

nyone can tear apart an engine down to its bones — block, heads,
manifolds, etc. — but it takes a pro to complete the next step:
machine work.
Machine work is necessary to ensure the engine block, heads and
other external engine parts have flush surfaces, such as manifold
openings, so that these parts can be reunited without fluid or air leaks.
Machine work also opens such areas as cylinder walls, crank bearing
openings and other spaces within the engine to an exact dimension that
the rebuilder chooses (much larger dimensions for performance
applications, slightly larger for stock applications). This allows the
moving internal parts of an engine (pistons, crank, etc.) to be installed
within these areas to an exact gap or tolerance, measured in thousandths
of an inch. If the specific tolerances stated in factory manuals are not
followed, parts with too little gap may not move freely, while parts with
too large of a gap will move sloppily and cause damage.