What did I break?

Hey JO,
The pistons are more than likely fine. Think overheated valves and you'll be closer to the truth I'd bet. Pistons only cause the valves to tighten up if the two come in contact and that is usually catastrophic to the entire assembly.
Pull the heads and have them repaired, have a look in the cylinders for scuffing. If it ain't broke, don't fix it.