I will look later for you if you are interested, but a friend gave me this advice. Once the airplane is FINISHED, with all the servo's mounted, etc., and otherwise ready to fly, mount the engine with rubber bands. Make sure that the engine has the prop and spinner mounted so that it will be the correect weight.

Then, you can slide the engine assembly back and forth to help balance the model. You still have to put the engine bearers on early in the construction process (when they tell you to do so) so that the side-to-side spacing is correct.

Keep your throttle linkage a little long so that it will still fit if the engine goes way foreward. Trimming it to length afterward won't make a major weight difference.

Bob