JPM, I'm just thinking out loud here. I don't know if this is a good idea or not - some guys might chime in with better ideas.
If the engine is apart, and the piston is not at TDC, could you mount the head/piston upside down on a bench (it would have to be fully supported) and put a forked stick down inside the piston and gently tap it to see if it moved? The fork would have to be carved so that it straddled the connecting rod and the wrist pin and rest on the underside of the piston. The wood would have to be soft (pine?) and the tapping should be gentle. With a good sanding disk and a little creative X-Acto saw work, it would only take a few minutes.
I would NOT suggest hitting the connecting rod directly - there would likely be damage to the connecting rod and bearing, wrist pin, and piston journals if you did that.
Was there any trace of a fine, white powder inside the piston between the piston and the sleeve? It might look like dried salt? I am thinking that there could have been corrosion between the steel piston ring and the aluminum wall. If it had been in a damp environment, galvanic corrosion could have gotten in there and 'welded' the ring to the cylinder sleeve. But that would be in an extreme circumstance. I used to run into that in my old job (not in RC engines) quite a bit. It's a slim possibility that this might be your problem. I emphasize the word 'slim'.
Just my $.02
Bob
Last edited by N1EDM; 03-03-2015 at 04:09 AM.