Hi Thomas,
Greg has one of the most used ways, I use Marvels miracle oil and some
kerosene mixed. Let it stand for a week, then with the shifter in neutral,
use a 17mm open end wrench placed on the bolt head (pulley side) and move it
back and forth. Since you already have the head removed then tapping it a
bit will help. If it does not break loose the first time then put some more
mix in and let it set a few days more. What has happened is the rings
(ferrous metal) and the cylinders (also ferrous metal) have rusted together.
The rings have movement in the pistons because they are non-ferrous metal
and won't corrode the same as steel to steel will. Because the rings are
most likely free or have some movement in the inner edges, tapping the
piston can cause them to break. So rocking the crank shaft is the better
way and if it moves just a bit, well that's the rings shifting. Get the
idea? Move the crank shaft to the brace the rings then tap a bit if your
going to tap on the pistons at all. You should get your pistons free in a
few weeks, depending on the corrosion.
Bill