The Tower Trainer .40 MkII ARF is a very lightweight airframe, and it balances best with an engine in the 12 to 13 ounce range. Your optimal engine choices would include the O.S. Max .46 LA, Thunder Tiger GP-42, Thunder Tiger Pro .36, Magnum XLS .36, Evolution .36 NT, or Super Tigre G-34.

Ball Bearing .46 2-strokes normally weigh in around 16 to 19 ounces, and even with the receiver battery clear in the back of the fuselage behind the servo tray, the Tower Trainer .40 MkII ARF will come out nose heavy.

If you want to fly a ball bearing .46 2-stroke, please purchase the Tower Hobbies .60 MkII ARF. A ball bearing .46 is just about perfect for the larger trainer.

For the Tower Trainer .40 Mk II ARF, my personal choice would be the O.S. .46 LA or Thunder Tiger GP-42 with the "Nexstar" 11x5 prop from Hobbico. Either one of these engines are top quality and dead-solid reliable. Both will fly best with castor oil lubrication rather than glow fuel with 100% synthetic oil. If you only have 100% synthetic oil fuel available, then a .36 ball bearing engine (G-34 included) would be a better choice.

Good luck, and good shopping.