Just getting back into R/C after MANY_MANY years and would
like to "recycle" some of my old stuff. Do you think I can use
an older .61 2 stroke OS engine on a modern flat bottom wing trainer that calls for a .40-.46. I dont think that the weight difference would be enough to cause problems as long as the c/g is correct. I realize that I'll have to go easy with the throttle
but will I be able to fly it without too much hassle?
Thanks

Why not pick up a 60-size trainer? If you go with the smaller one, you'll likely have to use some bigger wheels in order to get enough ground clearance for the prop. I think your older .61 will be at least a few ounces heavier than a modern .40. Most of them are well under 20oz with the muffler now.

I was given a new unassembled "40" size trainer and I was hoping to use an engine that I had from my "OLD" days of flying.
As I said I'm getting back to the hobby after many years. I have
all sorts of bits and pieces left from the old days and would like
to use some of them to get into the air without buying more stuff.
Bob

If you've already got the trainer, you might as well use it.

Install the engine you have and go fly,if prop is to close to ground then install taller wheels.
Have a goodun,John.

Thanks, hope I can get it all together before the snow flies. :-(
Bob