*.*The brass coupling is a through hole connecting two optical axes of different diameters, or connecting motors and an optical axis, or connecting axes of different diameters.*It can be used as either a screw rod or a transmission between the propeller shaft and the engine of a model remote control ship.*Side screw holes are unified as M3. Each coupling is supplied with four screws.*There are many children of joint couples on sold. For example "2mm to 5mm" means the apertures diameter at both ends were 2MM and 5MM respectively. Please choose the correct sizes to purchase.Specifications:*Material: brass*Diameter of both ends of cross section: 9mm*Intermediate diameter of cross section: 7mm*Total length: approx.20mm*Optional sizes:2mm to 2mm2mm to 2.3mm2mm to 3mm2mm to 3.17mm2mm to 4mm2mm to 5mm2mm to 6mm2.3mm to 2.3mm2.3mm to 3mm2.3mm to 4mm3mm to 3mm3mm to 3.17mm3mm to 4mm3mm to 5mm3mm to 6mm3.17mm to 3.17mm3.17mm to 4mm3.17mm to 5mm4mm to 4mm4mm to 5mm4mm to 6mm5mm to 5mm5mm to 6mm6mm to 6mmPackage Contents:1 x brass coupling4 x screws1 x wrench