As another old fart engineer I would like to add by 2 bobs worth to the discussion. . I have now idea of Brian's experiences except he produces some damn fine models and his posts have helped me in the past. To the problem of fuel feed and raising the tank it sound to me to be about basic fluid flow. This type of problem is found all through the world of pumps and pipes. An easy fix is to increase the fuel pipe ID and that of the check valve. I have had these problems in water supply to boiler pumps. Each small elevation of the fuel tank has the equivalent of enlarging the pipe ID. Every fitting or bend in the fuel line has the equivalent of lowering the pipe ID. Apologies to those who already know all this, PeterThis post is not so much about rings, but it does show something very important. I am always amazed at how much a difference in gas tank height affects an engine. When I built this engine a few years ago, I had made a gas tank to fit underneath the cylinder, with a 3/32" diameter check valve in the discharge. This year when I went to start it, I couldn't get it to draw fuel up from the tank. Okay, when these engines set around on the shelf for years, check valves have a tendency to freeze up. That wasn't a big deal, I just grabbed a spare gas tank I had and mounted it on a block of wood. This allowed me to start the engine and see that it actually did run. After putting the cast iron rings in, the engine would run good for four or five minutes and then die, as if it were running out of fuel. When everything else is set at "optimum" and the engine dies for no good reason, I always suspect the gas tank height. In this picture you see a 1.6" tall aluminum spacer under the tank. That fixed it!! Now the engine will run until I turn it off with the switch in the electrical system.