Irritatingly, whenever I try to make a new extruder using my existing one, it always breaks down forcing me to have to repair it, even though it is about to be made obsolete. It is as if they know!
The GM3 motor on my extruder started making a noise like a machine gun. On opening it up I found it has stripped a tooth of the final gear.
Since I moved from the 6V version to the 12V version I have been getting pretty good motor life. I do have to lock the clutch and sometimes glue the splined shaft into the last gear, but so far the gearbox has lasted well.
It is just as well my next extruder uses a stepper motor and an all metal drive chain: -
More details soon ...