Mostly Stephenson valve gear was used for this, but then four excentrics on the main shaft are required. Using lego it would result in a weak shaft. The Maudsley gear used on this engine avoids this , also it makes it very easy to adjust the valve timing.
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These types of files are created with the LDraw system of software. You will need to install compatible software to open and view the instructions.
A full list of available software is on the LDraw site.
This MOC also has a LEGO Digital Designer file available: 3D Model