4006 Series Hyper Coupler (6mm D-Bore to 5mm Hex Bore) - goBILDA
uxcell Hex Coupler 4mm Bore Motor Hex Brass Shaft Coupling Flexible Connector for Car Wheels Tires Shaft Motor 2pcs - Amazon.com
Precision sleeve couplings now include hex, square, and threaded bores
Amazon.com: HJ Garden 4pcs 6mm Brass Hex Shaft Coupling Motor Flexible Coupler Joint for Connecting DC Motor Shaft, Robot, Model Car Tyre etc. : Industrial & Scientific