This servo/gear reduction uses mostly 3D-printed parts. The servo uses a 775 36V 9000rpm brushed DC-motor wich is driven by a BTS7960B motor driver wich is controlled by an arduino mega 2560. The stall torque of the servo is about 55kg/cm wich is about 5.39 Nm. the peak current is about 18 amps when using a 6s LIPO battery (about 22-24V). the reduction ratio is 1:30. I originally wanted to use some 20mm ball bearings for the outputshaft but they didn't arrive in time so I ended up using two 20mm holes and some lubrication so the shaft can rotate freely. For the intend of filming the internals of the servo for possible faillures, I printed some collored gears and lasercut a 5 mm acrylic lid. Unfortunatly I didn't get to film any awsome footage of gears exploding because they were far to strong ;-). Even at a peak torque of 54kg/cm they didn't show the slightest form of flexing or cracks. The two middle gears (blue and red one) use a stainless steel shaft of 6mm diameter. The holes of the gears are designed at 6.2mm diameter so they would fit nicely around the 6mm shaft but some additional filing/sanding might be required. The same might has to be done for the 20mm shaft holes. 

STL-files can be found here: https://www.thingiverse.com/thing:3292860

Fusion 360 source files can be found here: https://www.cgtrader.com/3d-print-models/hobby-diy/robotics/3d-printable-high-torque-servo-gear-reduction

The video can be found here: