Never tried a 12x6 on the OS 50 SX. As I said in my previous post, the 50 SX doesn't like to turn larger diameters with high rpms. It will certainly run but don't expect too much. There is a reason OS take this engine off the line. Try the 55 AX with the bigger prop. If you want to go speed don't waste your time with OS. Go Jett, Rossi or MVVS.