Also it's customary for glow engines to be measured in cubic inches and gassers to be in cubic centimeters. I don't know why that is, but that's what I see. So if the box says the plane needs a .60-.90 engine or simply says .60 size, that means a 2 stroke glow engine. As stated above, 4 stroke glow engines need to be a little bigger to give adequate power, so a .60 size sport plane would fly well with an .80-1.00 4 stroke. When choosing an engine, go with the lower end for the lightest weight and for smooth sport flying and longest flight times with a given tank, and go up from there to get more vertical performance or more straight line speed.