Some fliers here use a pipe and 9X6 prop on the FX turning it up quite a bit over 13.3k, and 5 years later, still running strong. I've got at least three Rossi .45 engines that routinely turn 16,500 rpm with 9X6 props too, and perform like new. Short lived engines isn't from RPM as much as a lean run, dirt/FL sand sucked inside the carb, lack of after run care, and of course, crashes. OS rates the .46 FX and AX to withstand RPM over 13.3k...GMS .47 even lists the 9X6 prop on the tower site for pylon applications. Proper fuel, care, and mixture, the engines should outlive us.