Belted extruder
Gear 3:1
Easy swapable filamentsize by just changing 1 (front) part.

Additional parts:
2 pcs ballbearings 695zz: 5x13x4
1 pc screw M5x35 hex-head
1 pc lock nut M5
1 Extruder Driver Gear Wheel 40Teeth 11mm Bore 5mm (MK7 / MK8)
( https://www.banggood.com/de/2Pcs-MK7-MK8-Extruder-Driver-Feeder-Gear-Wheel-40Teeth-11mm-Bore-5mm-For-3D-Printer-p-1036149.html )
1 pc ballbearing V-nut 623VV (1.75mm filamentsize)
1 pc set screw M3x12 (axis of ballbearing)
1 pc ballbearing V-nut 624VV (2.85mm filamentsize)
1 pc set screw M4x12 (axis of ballbearing)
9 pcs M3x25
2 pcs M3X45
6 pcs square-nut M3
1 pc hex-nut M3 ( locknut )
4 pcs spring https://www.banggood.com/de/10pcs-Spring-For-3D-Printer-Extruder-Heated-Bed-p-995375.html?rmmds=search
2 pcs bowden coupler M6 ( for 1.75mm filament )
2 pcs bowden coupler M6 ( for 2.85mm filament )
1 pc motor NEMA17
1 pc pulley 20T
1 pc closed belt GT2 90T