A small 4:1 gearbox. Useful for hobby or teaching. The gearbox has an open lid so watch your fingers. Have included a coupling that fits tight on a 2.3mm motor shaft and slips over the gearbox input shafts. I printed the big gear with supports and the rest without.All you need are two self tapping screws. I used 10mm long with a 6mm head. I also gave the gears and shafts a light greasing to stop them squeaking. Had it running nicely at over 4000rpm input so watch those fingers.