i had a somethin extra with a 70 magnum fourstroke the prop i had to swing was 12x8 that is the smallest prop magnum said i could use the pitch was way to much but kept the engine in its range with over revving sorry to say it didnt make it through the first flight with this engine the speed it got at 3/8 trottle off the pitch of the prop was more thasn it could handle it got a flutter and went in i had flown it several times with a 45 magnum pro on it it was a great flyer but if i was going to put a four stroke i would get a taller gear so i couyld run the biger less pitch prop that is just my opinion with the past experience, their a great plane that fly great i am know building another one out of coroplast hopefully it will fly just as good .