Aopin 4mm to 4mm Rigid Couplings Set Screw Shaft Stepper Motor, Length 25mm / 1" Motor Coupler
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DC12V Electric Gear Motor Micro High Torque Reduction Motor with Centric Output Shaft 4mm Dia for RC Car Robot Model DIY Engine Toys (30 RPM) : Amazon.ca: Toys & Games
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4mm to 8mm Rotatable Universal Joint Coupling Shaft Stepper Motor, Length 35mm / 1.38", Coupler Connector with Screw for 3D Printer, RC Robot, Car Boat Shaft, DIY Encoder, CNC Machine - Walmart.com
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4mm, 10mm, 20mm Micro Size Stepper Gear Motor Planetary Gearbox with Metal Shaft 3V, 9V, 12V 24V DC for Robot Joint - China Gear Motor, Geared Motor | Made-in-China.com