The 2″ diameter pistons were turned from gray cast iron.

To ensure concentricity, each piston was mounted on its piston rod, chucked up and turned to the final diameter.
 

Piston ring grooves turned with an indexable cut-off tool. Since my lathe is not capable of turning slowly enough, I fabricated a crank for the lathe and slowly hand-turned the spindle while advancing the tool to avoid chatter.
 

It took some time but yielded good results. Bonus: it was also a pretty good workout for my left arm!
 

Cast iron piston rings were fitted and compressed with a hose clamp and slathered with oil in order to slide down into the cylinders.