As geezer suggested....measure everything while its easy and out of the engine....
I still dont believe carbon build up would cause a knock you could hear at idle...I would be double checking the small end and piston pin bore sizes....
I had a six cylinder a long time ago that had a distinct knock at idle...turned out the piston pin had galled the indies of the piston bore when assembled...it was only when I pulled the pins out and measured the bores etc that I found it.
The used rings have been through multiple heat cycles and are usually run hardened after 1000 miles or so....8000 miles...way too many cycles....new rings are cheap insurance...but make sure the end gaps are perfect.
Also check the ring land clearance with the ring that will be used in that ring land and piston.
