FUSHIBEARING 2PCS Aluminium Plum Flexible Shaft Coupling Diameter 20mm Length 25mm Connector Flexible Coupler for 3D Printer CNC Machine and Servo Stepped Motor (4-4mm): Amazon.com: Industrial & Scientific
Amazon.com: Great Planes C-Clip for 4mm Rimfire Motor Shafts (10) : Toys & Games
X-DREE 3,175mm bis 4mm Motorwelle Universalgelenkverbinder für DIY RC Modell Spielzeug(3.175mm a 4mm Conector Universal de Eje del Motor para DIY RC Modelo de Juguete : Amazon.de: Toys
Interlocking Rings 4mm-25mm DIN 705 incl thread bolt DIN 914 Wave Rings Stainless Steel A2 | eBay