I ran across this fun video from Uncle Tony's Garage. Here he and a helper do a side by side test of different solvents to free stuck piston rings. They used 4 loose pistons from a core Slant Six engine. It's interesting and fun--the winner might surprise you, it sure did me!
BTW- This is NOT a method to use on a complete running engine!!
Founder and Constant Aide to Gadgetman
Gadgetman Reno, NV