I learned to fly on the Nexstar Select RTF with the O.S. Max .46 FXi engine included with the package. The Nexstar is actually closer in size and weight to a typical .60-sized trainer than it is to a typical .40-sized trainer. I think a ball-bearing .46-sized glow engine is probably the best fit, and you don't have to spend a nickel more to get one than you would pay for the O.S. .40 LA:

http://www2.towerhobbies.com/cgi-bin...?&I=LXBUX6&P=0

The Magnum XLS .46A is a powerful, reliable ABC 2-stroke and Tower has it on sale right now for $59.99. This would be a great performer on the front of a Nexstar ARF.

Another fantastic trainer ARF and engine combination to consider would be the Tower Trainer .40 MkII ARF at $69.99 and the Super Tigre GS-40 ringed 2-stroke engine for $49.99. The Tower Trainer .40 MkII ARF is made by the same folks who make the Nexstar. I'm building one right now and the engine mount and fuel tank for the Tower Trainer are identical to my Nexstar. The Tower trainer is lighter weight, so the Super Tigre .40 would pull it around with authority.

I'm using my old .46 FXi on my Tower Trainer. I know it doesn't need that much power, but since I already own the engine, I figured I'd use it. If I needed a motor for my Tower trainer, I'd purchase the Super Tigre GS-40 for it.

The $40 you'll save by selecting the Tower Trainer .40 MkII ARF and Super Tigre GS-40 engine versus the Nexstar ARF and the Magnum XLS .46A engine will pay for 1/3 of your radio system. Either combination would make for a great trainer and would be fun to fly.

For the record, the O.S. Max .40 LA is a great engine and will provide many years of reliable service. Engines with bushings instead of bearings benefit from being flown with extra Castor oil, and most of the fuel at your local hobby store is 80% synthetic oil and only 20% castor. The folks who don't like their LA-series engines would probably like them more if they ran the proper fuel mixture. An LA-series engine should be flown with 50% synthetic/50% castor fuel at the minimum, and if you can find 100% castor oil fuel that would be better still. The heavy viscosity (thickness) of the castor oil helps the bushings support the engine crank.

A .40 LA on the Tower Trainer .40 MKII ARF and the right fuel mixture would be a good combination, but the Nexstar really needs the extra power since it's about 1 to 1.5 lbs. heavier. It's simply the difference between selecting a 5lb. trainer or a 6.5 lb. trainer.