Mechanical

The Torsion spring needed to join the shoulder piece with the arm bottom.

I found a coil printed in PLA with a rod diameter of 3mm and a total height of around 10mm works. (it needs not be too precise)

I also experimented with creating a torsion spring from using a bending guide on some brass rod (see picture).

I ended up going with the PLA spring as it was easier to fit, but if you want a stronger spring brass rod is definitely the way to go.

In the assembly of the shoulder, there is a part listed as 120mm half bearing. It's really just a 120x90x5mm ring. I printed this in PLA.