A bit of explanation now, on what I've been doing. I was very surprised when the engine didn't want to run with the new cast iron rings I made for it. And then, I figured out that I had used the wrong kind of iron to make the rings from. This was surprising, because the rings were made from the same stick of iron that made two or three sets of rings that did work in the engines they were made for. The shop I buy my iron from didn't seem to know if this was ductile iron or the other type of iron. I found out after the fact by seeing how easily the rings bend. My current piston is aluminum and is 0.873" diameter, riding in a cast iron cylinder. When the engine wouldn't run, I made a second aluminum piston the same diameter with a viton ring for a reality check, and the engine ran fine with a viton ring. I then ordered a stick of the correct type of iron for making cast iron rings from Wisconsin, and it showed up here yesterday. I have a choice now--I can make new rings and put them on the existing aluminum piston----or---I can make a new cast iron piston which is a closer fit in the cylinder and put new cast iron rings on it. This is why I made the brass external lap, to lap the new cast iron piston so it would be a closer fit in the cylinder.-not an air tight seal, but much closer to it than the aluminum piston was. I'm not worried about thermal expansion, because the piston and cylinder will be the same material so if the piston grows a bit from heat, the cylinder will grow as well so it shouldn't seize on the piston. At the same time I was waiting for my new iron, I started a new thread about the possibility of using my Toolpost Grinder from Little Machine shop to grind the piston to the exact outside diameter instead of lapping it. I got so many varied answers to that question that I decided not to do it. So, armed with the knowledge I have collected, I will now make a new lapped cast iron piston from the old stick of cast iron from my local supplier and new cast iron rings from the new correct material from Wisconsin.