G'day all - I agree with w8ye - a 9 x 6 is ok to run the engine in on but once it is run in a 10 x 6 or 10 x 5 is better. I have had several ASP/Magnum 30s and they all took a while to run in but once they were, then the larger prop was the way to go. I am currently running a Saito 30 in a Tiger Moth which was designed for a 40 to 50 four stroke. The little Saito 30 is fine. As it was new, I tried a 9 x 6 on its first flight and it was very underpowered but the next flight, I used a Bolly 10.5 x 5 and it was much much better. I also have a couple of OS 26 four strokes. I always used to use them with a 10 x 4 (on a Dynaflite Butterfly) but lately I tried a 9 x 6 and it was not as good as the larger prop. A friend runs an OS 26 in an Airsail Beaver with a 10 x 6. Perfect. These little four strokes will swing larger props than most people think.