I break in my glow engines at SCREAMING RPMs with very small props. I want my motor running at the manufacturers MAX rated RPM while it's breaking in. I also want it plenty rich. Not rich enough to 4-stroke, but rich enough that it's belching a lot of smoke and oil out the exhaust pipe. It takes a small prop to get those kinds of RPMs and still be rich.

I use 4 or 5 props as I break in an engine. Start with the smallest one and work my way up to the big one. Lean a few clicks per tank and keep the RPMs screaming.

I think it works. I once broke in a 60 size motor with the prop I intended to fly on it. I was a newbie to the hobby and thats what the guys at the field told me to do. That caused a LOT of load and heat on a brand new engine. That engine never did run very good. It started and ran, but it was no racehorse. I even went down to a 10-6 prop on that motor later on just to see if it would pick up RPM. It never would turn over 8800RPM, no matter WHAT prop you put on it.

Break em in screaming and rich. They'll always be powerful. Your mileage may vary.

Flame suit on.