I've been playing with the simulator Donrecardo posted. I managed to scale up the Full Double Oscillator model to near where I want to build it. It brought up a question. The default model shows the piston covering half the ports at TDC and BDC. What effect would this have on a running engine? Should I modify my design to not cover the ports? To cover them less, more? What affect would this have on a running engine? If this was covered in the posts previous to this, I missed it. Bear in mind, I have no experience running my engines on steam. I might build a boiler some day, but for now I will run them on compressed air. So explanations should be primarily for air, with maybe a contrast if run on steam.