I just noticed on one of my CS2's the piston only has one plastic bearing ring. The other has two, and the drawing shows two. Well color me green! now I know why that one compressor always was louder and had more vibration than the other. Not a huge difference, but quite noticeable when you're looking for it. I added a second ring from my parts stash and now it's as quiet as the other. Came that way from the factory, but it didn't do any damage to the cylinder, as it looks new. The support from the high pressure piston must have saved it.
Anyway, something to look for if yours is acting funny.
Anyway, something to look for if yours is acting funny.