I’m assembling the extruder I do have and I ran into this problem, the bore of the extruder idler lever is too small to fit over the stepper motor shaft. I’ve never assembled one before but according to the instructions the stepper motor should spin freely after assembly and this will not.
Try to rotate the large acetal gear to see if it moves smoothly.
Do I use a drill bit to hone out the bore or do I get a new extruder idler lever that works with a 5mm stepper motor shaft? [attachment=1]extruder_lever.jpg[/attachment][attachment=0]stepper_shaft.jpg[/attachment]
Having measured my stock here. It appears your motor shaft dimensions are slightly out. My extruder idler arm measure 4.95 and motor shaft 4.95. If its not too difficult, can you swap that motor for another?