It occurred to me that in a single acting engine the valve and piston could be hooked up so they have the same travel (and travel direction) if the valve was something like the attached, that is, a rod with about half cut away like a d-bit to allow the escape of steam with a transverse hole to admit steam at the appropriate time (valve in grey, piston in dirty mustard colour!). Is there anything obvious I've missed here, it seems too simple to work!
TIA
Paul