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Just as I was reaching a point where I didn't want to go any farther without more knowledge about the cylinder and valve, Julius from New Zealand has came thru with a bunch of drawings and saved me. Thank you Julius, from the far side of the world!!!
I have seen in the flesh finished examples of this model, which is based on the surviving original engine in The Science Museum (London). The model has the correct type of rotary 4-way valve.The video in post no 9 is particularly unfortunate, as it goes to great lengths to promulgate a completely false idea of how Trevithick's engines worked, based on Julius's invention of a shuttle valve.http://www.brunell.com/product.asp?cookiecheck=yes&P_ID=135&numLanguageID=1