I knocked this up over a weekend to see if it was possible to convert an old 4-stroke petrol engine into a single-acting steam engine. It turned out to work pretty well so I'll probably use this technique as the basis for future steam engine work. You have a crank, conrod and piston already there so there's no need to re-make them!
The engine is of the inverted-valve type. I removed the valves and fitted two little brass plugs to block up the valve ports. I then made a piston-valve arrangement which screws into the sparkplug hole. This was made from copper plumbing pipe.
The piston valve is driven from a rather elaborate system of levers from the crankshaft.
Running from compressed air at about 50psi, the engine runs about as fast as it did when it was a petrol engine which is rather impressive!
The two photos at the end are of a couple of Clarke air compressor pumps which you can buy from Machine Mart (UK) for just over £100. Quite a bargain considering they're solid cast iron! It would seem that the obvious thing to do would be to convert one of these beauties to a steam engine. You've even got a twin-V engine!!
15/09/13: I dug this out of the shed recently - it still runs, so I did a video of it:
30/08/15: In response to a viewer inquiry, here's some photos of the actual slide valve used in the engine:
I didn't use any sort of sealing ring on the valves, since that would tend to catch in the ports on the side of the tube. The valve discs are aluminium, 10mm thick and about 20mm diameter. This is a close sliding fit in the copper tube - the 10mm thickness provides enough of a seal to prevent too much air or steam escaping. The two discs are mounted on a 8mm stainless rod with turned and threaded ends. The rear disc is retained by the clevis joint thing. It would be possible to use a rigid sealing ring (like in a car piston) if there were bars across the ports to prevent the ring catching - probably easy to do if the entire body was a casting.