Dave, You are correct, it is the Differential engine I was thinking about. Another attempt at circumventing patents was the "Epicyclical" steam engine. This was invented, I believe to get around Watts' patent on the crankshaft and connecting rod. I have a model here at the house that our son built a few years ago. Runs quite well, but looks weird as all get out. Jesse in too warm for November W Tennessee USA. MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.