Gearbox for motor RS755 and RS775 Abdiel Carreño

Reduce high speed from the motor (DC 24V: 34000 rpm).
Compatible Models: 755,775.
Reduction aprox.: 12,7:1
Torque With DC Motor 775: 20kg.cm
It will be use on Robot Sumo. Battlebot.
It was printed with PLA around 6 hours.
Test
https://youtu.be/E07UPMWj7z0
You need :
2 screws for attaching the engine
5 washers  
1 motor model DC 755, 775
http://www.mabuchi-motor.co.jp/cgi-bin/catalog/e_catalog.cgi?CAT_ID=rs_775vcwc 
4 screws for top 3mm
Two rods or axles 3mm diameter
You can buy the things here
Metal pinion: http://es.aliexpress.com/item/2pcs-Motor-Metal-Steel-Gear-Wheel-Top-Screws-15-Teeth-Tooth-5mm-Hole-D...

Motor DC755
http://es.aliexpress.com/store/product/3V-24V-4700-39000rpm-the-length-of-shaft-24mm-bassinet-DC-gea...