The piston is made from cast iron, exactly the same as you'll find in some of the latest Enyas like the 40SS. The liner is made from steel the same as in a ringed engine. I have to disagree with the running in method though because you should never run them out of a 4 stroke for some time. Initially the piston needs to stay as cool as possible until the liner is run in and then start the heat cycling and gradually leaning out to get the piston run in and the cast iron stabilised. The weak point with the S35 is the lower conrod bearing which isn't bushed and absolutely needs a minimum of 25% all castor.