Although the Zenoah G-23, from my measurements has an imperial sized nut on the drive shaft (1/2" tool required to tighten the nut). I strongly suspect that this adaptor can work (if youre not too fussy... since the size of the actaul drive shaft is 5/16"(7.94mm), and threads of 20tpi(1.270mm). In otherwords its an "equivalent" metric substitute. I am purchased it so I'll let you know how it works out...cheers.
Sorry for the long wait I had to actually receive the nut first (don't forget its hobbyking it takes a while sometimes).
This nut fits perfect. I've also taken some more precise measurements on the zenoah g-23 and the shaft is actually the exact same as this nut (M8x1.25). Considering my prior experience as a machinist I should have figured this right off the bat, judging from the fact that the Zenoah engine is Japanese (Komatsu), which is METRIC.
Ok its exactly as described, brass nut with specifics of M8x1.25, but dont forget that the allen screw which actaully holds the spinner in place is 3mm diameter. Its a funny nut too the tool used to tighten it unto the prop shaft is 1/2 inch.