RE: ringed engine break in?
All of you:
Listen up, guys. Millionth inch accuracy has been available since the 1930s. Do a little research on grinding the mirror for the Hale telescope on Mount Palomar. They used optical interferometers to check the grinding, and later, the polish.
Those close tolerances weren't commonly used because the instruments were expensive, and that that much precision wasn't necessary in everyday things. Now that lasers and greater precision tools are available we see many things decreasing in size, and reliability increasing because of the more consistent manufacturing.
Just consider a modern computer with the CPU running at a clock speed of 3 GHz and higher - some of the internal conductor strips are only two or three ATOMS wide.
Bill.