. The brass coupling is a through-hole connector designed to join two optical shafts of different diameters, connect motors to a shaft, or link shafts of varying sizes. It can be used as a screw rod or as a transmission component between the prler shaft and the engine in a model remote control boat. Side screw holes are standardized as M3. Each coupling comes with four screws. Multiple size options are available. For example, "2mm to 5mm" indicates the diameters of the two ends are 2mm and 5mm respectively. Please select the appropriate size for your needs. Specifications: Material: Brass Diameter of both ends (cross section): 9mm Intermediate diameter (cross section): 7mm Total length: Approximately 20mm Optional sizes: 2mm to 2mm 2mm to 2.3mm 2mm to 3mm 2mm to 3.17mm 2mm to 4mm 2mm to 5mm 2mm to 6mm 2.3mm to 2.3mm 2.3mm to 3mm 2.3mm to 4mm 3mm to 3mm 3mm to 3.17mm 3mm to 4mm 3mm to 5mm 3mm to 6mm 3.17mm to 3.17mm 3.17mm to 4mm 3.17mm to 5mm 4mm to 4mm 4mm to 5mm 4mm to 6mm 5mm to 5mm 5mm to 6mm 6mm to 6mm Package Contents: 1 x Brass Coupling 4 x Screws 1 x Wrench