MAX-25FXII 4.07cc 2-Stroke Engine w/ SilencerO. S. Engine MAX 25FX II 4. 07cc 2 Stroke Airplane Engine The O. S. Engine 25FX II is a high performance side exhaust aero 2 stroke engine that is suitable for many styles of small R C aircraft such as sport, scale, aerobatic and even . 25 class pylon racers. The 25FX II uses a direct needle 21L carburetor for excellent usability and a 892 silencer with adjustable exhaust direction. O. S.'s legendary standard of build quality is evident throughout.