The Ender-3 Pro Metal Y Belt Tensioner does not have properly fitting washers, leaving a space next to the belt idler. This causes the idler, actually made of two adjacent bearings, to separate in half. This washer fills that space to prevent this issue.
The inside diameter is 3.8mm, The outside diameter is 7.3mm. The height is 1.5mm.
It should fit perfectly. If not, try adjusting your Horizontal Expansion setting for this print.
If you want to remix it, find it on Tinkercad here
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