Thanks, Chuck. I first saw a small sketch of this engine in a model steam engine book I bought at the Kewbridge museum in London probably 30 years ago.
https://www.homemodelenginemachinist.com/threads/henry-maudslay-steam-engine.1109/
For many years I looked for other examples of the engine on the internet without success. Then, a couple of years ago I found a picture of a model which had been built many years ago.
https://www.bonhams.com/auctions/23024/lot/79/
With a 15 inch flywheel, it was about twice as big as mine and sold at auction for some $35,000. Some years ago I had created some drawings of the engine, not knowing any of the dimensions, but, by studying the relationships of the parts to each other, was able to get a close approximation. Then, several months ago I found a picture of a single cylinder model of the same engine.
https://www.gracesguide.co.uk/File:JD_2016_J_Spiller.jpg
With my mill/drill converted to CNC, I decided I could probably pull off the single cylinder version. In the model pictured, the cylinder and crank are mounted outside the frame. I changed the design of mine to be inside the frame. Fortunately, I had cast iron flywheel that was only a little too big and much too thick in cross section, so I thinned out the rim on my lathe and thinned out the spokes on my CNC mill. I also used CNC to cut the circle design brass insert, the feet, and the brass bridge part that supports the crankshaft. The frame sides were assembled from some 12 - 15 parts soft soldered together.
I never knew for sure how the valve mechanism was supposed to work, but came up with my own design which is probably close. The original drawing and models I found used an eccentric to supplement the cylinder motion, but I was able to design the stroke such that the cylinder motion alone was able to turn the valve enough without the eccentric.
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