I find a real keeper of a video over in the wasteland of utube. The Story Of Perfect Circle Piston Rings. I forgot how to link to it.
I think it's this one. The caption says it's from 1956, but it's definitely earlier. I also found one from 1974(?) that I didn't watch. These old films are great. New ones are ****py, don't explain processes and focus on how the company is saving the planet. Rather than give technical explanations and decent images, they present shots of cute children in a wild flower field blowing the floating seeds off of a dandelion head.
I was surprised that the rings were cast individually. I had thought that pipe like pieces would be cast, and the rings parted off from these tubes.