The engine worked in a Madness fun flyer. In this kind of model you can peak the engine to the best ground rpm because you need thrust at low speed and you don't want to fly very fast. This is the reason for the low pitch prop.
The OS Surpass 4-strokes rev up to 13000 with no problems. If the prop is to small they just dont turn more rpm.
I have yet to break one of these engines.
They are designed to turn in the range from 9000 to 13000.