First of all the casters I ordered from Amazon are much better than the plastic casters that come with the unit. I ordered the three inches that another user recommended. Some people have found it difficult to remove the black casters (which come with the unit), do this and you will be removing the old ones and putting on the new ones in 5 minutes.1. You will need a CLAMP WRENCH and a regular adjustable wrench or wrench that fits the nut.2. Using the VICE GRIP wrench, grasp and secure the top edge of the black wheel. It comes in an octagonal shape. This is the part that turns with the pulley. See photos3. Hold down the Vice Grip button. Using a standard adjustable wrench, remove the nut from the old pulley4. Remove the washer 5. Remove the black roller (immediately extends)6. Install the new roller 7. Replace the old roller 8 washer. You should be able to use the same nut9. ATTACH THE EDGE of the new roll with a 10mm wrench. Use an adjustable wrench to re-tighten the nut. Rinse, rinse, repeat for the second roller, done! Maximum 10 minutes. You must order two of these devices.