Engine breakin does not concern just the piston/ring/liner fit (or piston/liner fit in non-ringed engines). It also concerns the bearings, rod-to-crankshaft fit, and valve train in four-stroke engines.

I've been operating model engines since the late 1950's, and have been involved in model engine service for well over 20 years. I have worked with the designers of model engines from different manufacturers, and have literally run thousands of engines.

Ringed engines are typically run very, very rich. I don't know all of the thermodynamics involved, but I do know from practical experience that a breakin that involved some seriously-rich initial running, gradually working towards peak RPM insures that the engine will have good handling characteristics. Even newer engines that claim you need only one or two runs and go fly benefit from a longer breakin time. Different engines need different amounts of time before they really "come in". Once they have, though, they usually run very well.

I'll run the engine super rich with a fuel that has a castor/synthetic oil mix for about 15 minutes or so before I even think of leaning the enigne richer than 'four-cycle' mode. Then I'll gradually work the engine towards peak RPM in a rich-lean-rich-lean sequence, allowing it to warm up and cool off as the mixture is changed. A .40-.90 size engine will usually take a total of about 45 minutes of running time to get to the point where it can be leaned to just rich of peak RPM and hold the setting, except for the natural leaning that happens as the tank drains. Then I'll work on getting the idle RPM and mixture found. If the engine won't hold the needle setting, it's not ready to set an idle and fly. I'll richen it and work on it some more. Larger engines can take more time.

Four-stroke engines are run as rich as possible at full throttle without quitting...even if we have to leave the glow plug battery connected. This insures that plenty of lubrication is getting to the camshaft gears, lifters, and rocker arms. After 15-20 minutes of this, the mixture is then gradually leaned. The multi-cylinder engines get run this way for at least 20 minutes before we start to lean them.

With the procedures we use, and are written in almost all of the engine instructions, we have never had problems with an engine not reaching its proper performance. If it wouldn't perform, it was something else, and no amount of running would cure it.

The only time we've seen any true "glaze" (burned-in residues of any kind), have been in engines that have been seriously mistreated and overheated. We've never seen an engine damaged by the procedures we use. We have engines that have hundreds of runs on them with no damage, just normal wear. We have seen engines that have had sufficient use that the cylinder liner was polished very smooth. Those cylinders were usually worn to the point that they needed to be replaced. Scuffing them usually doesn't help that much.

What can I say. My experience has shown that an extended breakin for a ringed engine does no harm whatsoever...and is usually a better idea than trying to get the engine in the air as quickly as possible. I prefer bench running for breakin over flying rich because the engine can be under close observation and much better controlled.

My two cents. Take them how you wish.