Material: The 4 wheels are all made of super lightweight Aluminum. The belts are made of Premium Fiber Reinforced Rubber that is highly durable and longlasting. Specs: Big Pulley: 36 Teeth - 5mm Bore - Synchronous Wheel; Small Pulley: 20 Teeth - 5mm Bore - Synchronous Wheel; Belt: 200 x 6mm / 7.8 x 0.23inch Wide Application: Can be used in a very wide variaty of fields, such as 3-D Printing, Milling Machine, Gear shapeR, Drilling Machine, CNC Engine Lathe, Hobbing Machine or other Machinery Transmission Occasions. Set Contains: 2x Pulley Wheel 20 Teeth; 2x Pulley Wheel 36 Teeth - 5mm Bore; 2x Rubber Timing Belt, Length: 200 x 6mm / 7.8 x 0.23inch; 4x M4 Screws; 1x M4 Allen Hex Wrench Product provided by MMOBIEL - We always try our best to exceed your expectations at every turn. Join the ever growing list of satisfied customers and discover the difference with MMOBIEL.