i've bought some graupner propellers with M5 threads. my thought was to drill a hole in the center of a M5 bolt thats the same size as the motor shaft. then drill a M3 hole trough the head of the bolt to secure it with a setscrew against the motor bell. alternativly use loctite 648(gear/bearing glue) to glue the bolt to the shaft instead..
with internal M4 threads in the prop, there is barely enough (usually not enough) metal left when drilling it in center to 3mm.
with M4 props one could also glue inn a M4 nylon bolt in the prop, then drill it to 3mm(the remaining nylon will then hopefully stay in the prop threads. and then glue it with epoxy to the motor shaft.