"I had no problem with the plane being nose heavy. You just have to configure the plane correctly."

Yeah, by adding weight to it. Once you've moved your receiver and battery as far to the rear as is physically possible, that's pretty much the only option you have left.

The .46 LA would actually be a better choice for the Tower Trainer .40 than the .46 AX. It's much lighter weight (12.4 oz w/muffler versus 17.2 oz for the .46 AX) and it still has plenty of power for a trainer that is this light weight. There is no reason why a .46 LA should not last as long as a .46 AX either, as long as the recommended fuel is used. Bushings love castor oil.

I just built a Tower Trainer .40 MkII and I own 3 O.S. .46 FX/FXi engines. I think the O.S. .46 AX is a wonder engine, it's just too heavy for the application being discussed. The Tower Trainer .40 MkII is a light airframe compared to most .40-sized trainers with a similar wingspan, and it seems to balance best with a 12 oz or 13 oz engine.

There's no reason why you can't build a Tower Trainer .40 Mk II with a .46 AX. You'll either need to buy a 1200mah receiver pack for the plane or simply fly it nose heavy. I just think the plane would fly better if the weight/wing loading were kept down and it balanced out at the recommended CG point.

If you think you can build the Tower Trainer .40 MkII ARF with a .46 AX and balance it simply by moving a 600Mah receiver pack to the rear, you're going to be disappointed.