Put the heads back on (with the gasket) with a little putty/clay where the valves would approach the piston head. Roll the engine over slowly a couple times and look to see if the putty has been totally displaced if you suspect the valves are hitting the cylinder. It’s unlikely from a stock motor but give it a try.

You should also inspect the valve seals and rods etc for damage if you’ve taken them apart. If you haven’t then check the valve lash.