The main gear of my extruder drives the wood screw I am using as auger. During lots of experimentation, especially when failed designs lead to clogging of the extruder, there was a bit more wear on the shaft than it could handle. With a trick (placing an additional nut on top of the screw head) I could use that gear for a last print:
I have printed a copy of the failed gear to get my needed spare part.
Even though the result isn't perfect (I should have squared the weak printer mechanics before starting the print), the replacement part works fine: