You mentioned the break in period. Basically when sometbhing like an engine is machined and assembled the moving parts that contact each other are fairly rough even if they don't seem to be to the naked eye. As the engine "breaks in" these surfaces smooth out and the parts sort of mate together creating a very smooth mechanical system. Some engines need more break-in time than others. The OS .46 that I linked to earlier is machined very well and takes much less time to break in than some others.

I personally don't think breaking in an engine requires any sort of mystical voo-doo as some might suggest. Run a tank of fuel through it on the ground at a slightly rich carburetor setting then fly it a few times on the slightly rich side. The rich mixture helps keep the engine cool while the parts are mating to one another. After that, adjust the main needle and have a blast. You'll notice that the engine just seems to start easier and sound better after a few flights.