ORIGINAL: Missileman
Normal break in procceedure is more about seating the piston in an ABC engine. With the used engine I would just run it rich for a tank or two (I always run my engines slightly rich even after break in)
I follow the OS procceedure on my TT engines. Start engine, tune to high rpm for 5 seconds turn 1/2 turn rich for 20 seconds, reapeat and slightly increase high rpm time as you go for two tanks, then run rich for the first gallon of flying.
Some people just run a new engine rich and fly right away with no problems.
It's generally NOT recommended to run in an ABC engine with a rich setting. The cylinder is tapered to form a very tight fit at TDC. When the engine temperature comes up to normal, the brass cylinder expands more at the top to give you the fit you want. If it goes lean then the cylinder expands even more which prevents a seize-up.
Put a prop on about a size smaller than normal flight and start the engine. Bring it up to full speed immediately then back off perhaps a click or two of the needle. Many short runs with a cooling off period of 5-10 minutes is preferable to fewer but longer runs.