Lough Erne

meant to add It is roughly similar to this apart from the fact that the piston valves are above the main cylinders and as seen in the photo has an additional rocker assembly to lift the valve travel above the cylinders http://www.steamindex.com/locodesn/cylinders.htm

The piston valves are operated as mike says through a rocking shaft. From the eccentrics on the driving axle the expansion link is moved back and forth which in turn is suspended from the weight shaft. From the expansion link there is a connecting link which is supported on both sides of the frame spacer and this rocks back and forth. At the cylinder block side of the spacer as seen in the photo the connecting link is connected to the steel casting that the valve spindle link is connected to. This means as mike says that the valve is pushed/ pulled in the opposite direction of the throw of the eccentrics. I will take a few photos at the weekend and put them up

If you want any more photos feel free to ask and I will see what I can do

In theory an album should have appeared of 171