Package Includes: 10 Pieces Flexible Couplings 5mm to 8mm with 1 Allen Wrench. Size: Length = 25mm, Outer Diameter = 19mm, Inner Hole = 5mm. Quality Material: Made of premium aluminum alloy, it is durable with long-lasting use. Advantages: Flexible couplings can eliminate the stress between the motor and drive parts to reduce the imbalance and misalignment that occurs, and provide an extremely tight grip without damaging the shaft. Wide Application: Compatible with NEMA 17 stepper motors. Ideal for connecting different shaft diameters for RepRap 3D printers or small CNC machines, as well as servo motors, stepper motors, encoders, drive screws, machine platforms, etc. Specification: Product Name: Flexible Couplings 5mm to 8mm. Material: Premium aluminum alloy material. Color: Silver. Size: Length = 25mm Outer diameter = 19mm Inner hole = 5mm Package Includes: 10 Pieces Flexible Couplings 5mm to 8mm with 1 Allen Wrench.