I recently built the Tower Trainer .40 MkII ARF, and every engine mentioned so far would make it come out nose heavy. I know, because I put an O.S. .46 FX on mine, and it's nose heavy.

I'd actually recommend a ball-bearing .36 like the Evolution or Thunder Tiger Pro .36. A bushing-supported .40-size glow engine like the O.S. .40 LA or Thunder Tiger GP-42 would also be an excellent choice. The Tiger Trainer .40 MkII ARF is surprisingly lightweight, and any of these engines will power it with plenty of authority. My .46 FX is way more power than the plane needs, but it's what I happened to have on hand.

You mentioned you were looking at the "kit," which if you mean to glue the sticks together yourself is another matter. Kits will often turn out heaver than ARFs, and the kit has a shorter wingspan, so a .46 ball-bearing engine might be more appropriate if you are really planning on kit-building your trainer.

If you're planning on assembling the ARF version, there is one .40-sized ball bearing 2-stroke that appears to be lightweight enough to be a great fit for the Tower Trainer .40 Mk II, the Norvel AX-40. It ways about 4 ounces less than my .46 FX, so it will balance the plane much better. It is also available directly through Sig's website http://www.sigmfg.com for only $59.99 plus S&H.

A .46 ball-bearing 2-stroke in the 16 to 17 ounce weight range makes the Tower Trainer .40 MkII ARF nose heavy enough that I'm still considering buying a replacement engine in spite of the fact that I have two extra .46 FXi engines laying around and a GMS .47 still new in the box. If I don't buy a Norvel AX-40, my second choice would probably be the TT GP-42.