RE: Rules for 2 stroke engine's break-in
There are a couple of myths being perpetuated here but not to worry, they won't really harm the engine.
The first myth is about never running a new ABx so rich it 4 strokes. This doesn't do any damage at all but isn't quite lean enough to start the running in of the piston/liner. Optimal mixture is around where it's just breaking between the 4 and 2 stroking on the first tank or so. A healthy dose of added castor is a good idea though because it's very kind to the rod bushes.
The second myth is prop pitch. A fine pitch (lightly loaded) prop as mentioned by HighPlains cools the engine just as well as a coarse pitch. Because of an extended argument about this some time ago in RCU I carried out an experiment using a 10x8 APC prop and a slightly trimmed ~12x3 3/4 APC (trimmed to give an identical load) and there was no difference whatsoever in head temp. However for running in it's best to use a fairly small prop, the smallest diam/pitch recommended for a particular engine is fine although I go a bit smaller still.