this coupler fits to 8 mm shaft dia gear motor like Go-kart motor and square gear box motor.
its Diameter - 50 mm
shaft hole dia 8 mm.
Shaft hole shape same as the gear motor shaft shape D type. Its tight fit coupler. You need to use Araldite for more rigid joint.
Material type is PLA. with 100% infill.
 
Package Include- 
1 x Flange Coupler
4 x Nut & bolt set