I already ordered a set of new (to me) jugs, with good used pistons and rings, so I think I'll start there. This is so weird, I can't really explain what I saw. I still need to remove the valves from the head to make sure I didn't do something weird with a valve seat.
I did notice that the pistons have a good amount of play side to side on the wrist pin, but that should be normal, right? There's no detectable play up and down, just left and right on the rod, which allows the piston to center in the bore, right?
So damn weird. My brother and I installed the removed piston and its rings in the removed jug, and it fit snug as it should.