This is a 5mm (0.2″) to 8mm (0.31″) Aluminum coupler. A common application for this coupler is to connect a stepper motor with a 5mm (0.2″) axis to a larger 8mm (0.31″) rod. In 3D printers it is commonly used to opperate a threaded rod.This coupler is cut like a spring, giving it a flexible and spring property. SPECIFICATIONSouter diameter: 20mm (0.79″)Lenght: 25mm (0.98″)Fastening screws: 2 (1 for each side)Material: Aluminum SHIPPING LIST5 – 8 mm (0.2×0.31″) Coupling x1