It's probably 12mm. Can you measure the adapter in inches or mic it? 12mm = 0.47244 inches (472 thousanths).
ORIGINAL: por924
The spinner backplate must be reamed out to 12mm. The Fox reamer has the right size. The Fox reamer did the complete job for me without a drill. I still used it in a drill press. Also, I found that I had to ream my props for 10mm and ream the back half of them 12mm. I don't know why Moki machined their shaft that way.
I did all that too but I still need to drill/ream the prop for the adapter. What size is it?