UPDATE: There’s now a new update that replaces the new O-rings with washers. Read more here: https://investegate.de/prusa-xl-the-new-washers-for-the-pulleys/

Since September 2024, Prusa has released new O-rings for the Prusa XL belt idler pulleys.

So far, there is no official explanation from Prusa that I am aware of. After some research, I was able to find out more on Reddit and the Prusa forum.

Prusa has published newly adapted CoreXY parts on Printables. I compared the parts with the old parts and found that the new ones now have 1mm more clearance on the axis side.

The new O-rings are therefore approximately thicker than the old ones. The new O-rings have a thickness of 1.2mm.

Apparently, there were problems with the old O-rings and the insufficient clearance, which caused the O-rings to wear out quickly, resulting in uneven print quality and unwanted noise.

I and others have printed with the old O-rings for 1.5 years and have not noticed any errors.

However, it is recommended that you reprint the modified parts during your next extensive printer maintenance and integrate them together with the new O-rings. You can also maintain or, if necessary, replace the idler pulleys at the same time.

You may have noticed that there are two idler pulleys in the X-carriage, where the toothed side of the belt is guided over smooth idler pulleys. Why Prusa doesn’t use toothed idler pulleys for this is a mystery to many. This can cause the belt to deform. Therefore, it is recommended to use toothed idler pulleys. All parts and information can be found here: https://www.printables.com/model/1135912-prusa-xl-modified-corexy-parts-for-new-o-rings-wit