This stops the PTFE coupler from being lose and going back and forth. It has a thickness of 2.18 and fits on the coupler that comes off the extruder.

I am sure there are some of these out there but I was having trouble finding one so I just made it. I just started the “Maker Tales” FreeCAD tutorial and learned enough to make this. 

I used PLA with stock settings on an ender 3