CG retired, I just had an idea that maybe it's the shaft adaptor that's 5MM. I have a few of those with that diameter that the bore size of the prop is 5MM. The motor shaft is only 2.5MM.. I have never seen a small motor with the 5mm shaft. Especially on these smaller planes. I have one little plane with the motor shaft that's shaft is only 1.5MM It's really tiny.

Buying the right motor is a real learning process.