Someone mentioned the misuse of the term "clearance" and "tolerance" above.
Take note: they are VERY different animals.
Clearances are the gaps for the oil molecules to move in to prevent metal to metal contact.
Tolerance is the precision with which that clearance is produced and maintained.
In the 70s, those tolerances were such that most of the bearing width, for example, had the design clearances. The rest of the bearing width was either too tight (and would wear in) and too loose and servers no function in terms of bearing action.
Today, the clearances are maintained within millionth of inches across the entire bearing widths. No wearing in and full function of ALL of the bearing surfaces. In addition, the accuracy of surface roughness has improved out of sight.
That goes for pistons, bushes, mating surfaces etc. As a result, there is as good as no running in (wearing in) any more and engines last for 3 or 4 times as long as earlier ones with identical design clearances.
Oil development has kept pace with much less variation of molecule size and shape, reducing accidental metal metal contact for longer and longer periods.
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