OK folks, Lets hear what MVVS says about running in <their> engines. (they used to be squeecky tight when new)

Start the engine at half throttle, and run it four stroking rich. Every now and then, move up the throttle setting. If the engine sags, then go back to the half throttle rich setting running. The engine is run in enough for flight when it does not sag any more.

What does this tell us.
An ABC or ABN or whatever combination except steel and meehanite, will behave differently than what we old farths were used to.
Each manufacturer has his own material specifications which determine the amount of temperature rise for a given dimensional change.
In MVVS I found that a slight warming up resulted in loosing the very tight squeeking fit, which returned when the engine had cooled down almost to room temperature. This shows a very small temperature coefficient for the piston material. These engines will not be hurt by a slobbering rich running in. I even found, than when up in the air, and chasing hell, the engine could overheat and loose power, untul allowed to cool again by running at rich half throttle. Since no metal contact was present during these hot runs, no wear was evident after inspection. When run in well, this phenomenon disappeared.
It also seemed that the piston showed some "growth" during the first runs, and relieving the lower liner half by honing instantly restored full power on engines that showed longer times than the usual short sagging period during running in.
Other manufacturers probably use a different alloy, which will not allow this kind of running in, so I would be very carefull to not follow the manufacturers recommendations, be it slobbering rich, or just lean of that.
Know your engines, and believe what you see and experience. There is no magic involved. Just plain physics. Including the fact that a piston can expand the cylinder to a large extent when it rides on an oil film. Pinch start should be tested with the piston bone dry!
I also experienced the fit change by carelessly tightening the head screws. I now take my time to do it right.