TEVO Tornado, CR-10 and others similar printers come with a spring Stepper coupler and no Z axis rod subjetion, so when the printer push up the X axis with Z stepper motor, and there is any friction, the coupler spring tend to compress instead of rise the X axis. Later, when the coupler spring compresion reach a limit, the coupler free its energy, rising faster the X axis. The result is that this wobbling generates waves in our printed part horizontal layers, with some layer more compressed than expected and others expanded. In order to avoid this, this part allows an M5 screw with its nut to be placed on top of the Z axis lead screw, and one can tune extra compression on the Stepper spring coupler, so friction vanish and the print result in a even layer distribution in our part.
ABS is recommended for strength.