When I assembled the dual extruder a couple days back, one side was clearly acting in a slightly impaired manner. Inspection show that the M4 flat washer that seats against the printed idler arm had managed to work itself off the seat and was sitting diagonally. Possibly due to unfortunate tolerance stacking or possibly irreversible damage to the seat from it going sideways the first time, it would not stay square when reseated. As a quick and dirty I laid the washer semi-permanently in the idler shoulder using cyanoacrylate. My washer measured 8.8 mm OD which is a little small. I think a common M4 is 9 mm. The recess the washer sits in looks to be about 10 mm in diameter. I'd suggest that the recess be reduced 0.5 mm down to about 9.5 mm. That might eliminate this from happening to someone else.