What is the best way to clean the carbon crud off of my pistons and piston ring grooves?
A carbide piston ring groove cleaner with the right size cutter is the safe way to do the ring grooves.... it's a sealing surface and so needs to be cleaned very accurately.
For the face of the piston... I actually cheat and use a wire brush... but I'm very careful to get nowhere near the edge where I might scrape a groove in the side.
I have used the old rings to clean the ring grooves. You will need to file/grind and edge to be able to pick at it to get the crud off. If you did it right, you will have numerous cuts on your thumb
I must have done it very right.