RE: question about caliber 5 with 4 stroke engine ?
There are no 4 stroke heli engines that have enough power. I don't think O.S. even makes the 4 stroke that came in the Kyosho Nexus Legato a while back. From what I heard the helicopter was underpowered to begin with. The engine would have too large to get enough power, I don't think it would fit in your frames, if one even existed. Do a little reading on the Nexus Legato and I think you'll find that the 4 strokes just don't have the power to weight ratio needed for helis.