One of the issues I had with printing the knees was the long print time and a mistake I made when doing free form deformation scaling on the lower leg. I inadvertently scaled the knee disks and made them egg shaped instead of a perfect circle.

I cut the disks off with a Dremel just to salvage the print, and modelled my own disks with a captive nut inside for a 1/4-20 nut and bolt to act as a pivot point for the knee connection to the thigh.