What kind of fuel will you be using? Plain bearing engines like the .40 LA run best and produce more power when using glow fuel with large amounts of castor oil as part of the lubrication. If your local hobby store only stocks glow fuel with synthetic oil for lubrication, the Pro .46 will likely run better for you and provide simple, reliable operation.

Which engine balances the plane best? There is a difference of almost a quarter of a pound in weight between the .40 LA and the Pro .46 engines. It is likely that one of them, when test mounted on the plane, will balance your LT-40 on the recommended Center of Gravity (CG) better than the other. Starting out with an engine of the right weight will make it easier to trim and balance your plane.

The Kadet LT-40 is a terrific trainer and sport plane. Both the O.S. Max .40 LA and the Thunder Tiger Pro .46 are high quality and reliable engines. The .46 Pro will provide more power and it's extra weight will provide a bit of extra stability on windy days, but it will use fuel faster and increase the wing loading of the plane thus causing faster landings. The .40 LA will provide lighter wing loading and ensure smooth, gentle glide characteristics while training with the LT-40 and also sip fuel more slowly. It may lack the power to help you "punch out of trouble" if a landing approach goes badly, however.

Both engines are a terrific match for the Kadet LT-40, you can't really make a wrong choice, but there are a number of things to consider. If you want a light, gentle, "floaty" trainer and you can get castor oil fuel, then go with the .40 LA. If you want a faster, sporty trainer then use the Thunder Tiger Pro .46.

Good luck and have fun!