Every product has a unique Manufacturing Part Number label on the inner package that proves it has been qualified,which include Part Number,Model Number and inspection date information. Made of aluminum and rubber. The perimeter is 200mm, hole diameter is 6mm. Used with rotary encoders for the measurement of conveying speed and position. Size: 6.4 x 4.9 x 1.1cm/2.52 x 1.93 x 0.43 inch(OD x ID x T) Total Height: 1.7cm/0.67 inch Perimeter: 2cm/0.79 inch Bore Diameter: 0.6cm/0.24 inch Colour: black + silver gray Material: aluminum + rubber Package Contents: 3 x Encoder Wheel 3 x Wrench Note: If you have any questions about the item,please provide us the Manufacturing Part Number for checking. Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.