[Racer x]

I found the photo showing the rings. I will do some research and find the answer to your questions. My guess is they are steel shims. Meant to set the sleeves at the deck height. Yes dry ice will freeze a new sleeve very well. But only to put it into the block. Removal of the sleeve would require so much heat that trying to cool the sleeve in place would not be possible. And cooling the sleeve in the block would cool the block also and it would not release.
This whole process falls under the category of where angles fear to thread. I leave it to a machinest.