I have been reading a couple of articles by Dave Gierke ( Model Airplane News), one on Break-ins in general, another on 4 stroke. According to him, just running sloppy rich continually might not do the job. Quote: "Break-in requires short running periods during which the needle valve is occasionally leaned to a point where the engine comes up to speed and temperature, then is immediately richened to cool it down".

I have been trying this, and seems to work good. It's called "heat cycling", letting the parts heat up and cool down. The key word is "immediately". He also recommends no castor oil, which he says removes needed heat from the engine. I'm not sure about this, more study needed.

"For a ringed engine, there is a visible sign that break-in is probably complete, when the machine marks have disappeared from the visible edge of the compression ring as viewed through the open exhaust. "

The old lapped engines needed to be run very rich until the parts wore to an acceptable tolerance. We used to say a broke in engine was a worn out engine, not far from the truth. The newer engines have a more precise and consistent tolerance due to CNC, it's more a matter of relieving stress from the metals.

BO