In order to have an accurate conversation about valves and valve timing, a few things need to be defined, such as steam and exhaust port layout, valve inside and outside lap, angle of advance, and valve travel as defined by the eccentric throw.
Then the valve and eccentric have to be set correctly.
To discuss steam engine valves/ports as if they were all the same with the same settings really does not do justice to a true understanding of valve gear.
Not to be flippant, but it reminds me of a conversation with the wife.
She said "I saw a great car today", "really, what kind" I said.
"Blue" was the wife's response. Well that clears it up.
Pat J
Edit: There are four main valve events which define how the engine runs, and they are admission, cutoff, release and compression. Assuming a standard D-valve with outside admission, the admission and cutoff are determined by the outside lap, and the release and compression are determined by the inside edges of the valve. Inside lap is often zero. Outside lap and angle of advance for model engines is often zero. Without knowing the port/valve/eccentric throw/valve travel arrangement, any discussion of such has little or no meaning.
Edit02:
The drawings for Jerry's Beam engine above seem to show a typical valve/port arrangement for a model engine, ie: no inside or outside lap on the valve, and no cuttoff (steam is admitted to the cylinder for the entire stroke of the piston).
The angle of advance appears to be zero, ie: the valve is not admitting steam into the cylinder until the piston begins its downward stroke.
Some small models will not run with an angle of advance included, since the mass of the flywheel may be too small, and the piston will just stop before it reaches TDC.
The Jerry's Beam engine drawings also show an exhaust port equal in size to the steam ports, but typically the exhaust port is twice the width of the steam ports.
The Beam engine valve/port design is a safe design, ie: it will always work, but as engine size increases, generally a more accurate valve/port/eccentric design is desired since this will greatly affect engine performance and efficiency.
I think steam engine valve gear design is one of the least well understood topics that is discussed on forums.
If Brian's engine does not have the same valve/port/eccentric characteristics that Jerry's Beam engine does, than any comparison between the two is likely to be misleading at best.
Edit 03:
To answer Brian's original question of which way the engine will run depends on whether the valve is about to admit steam to the top or bottom of the cylinder when the crank begins to rotate the engine away from either TDC or BDC.
I believe large horizontal steam engines ran a certain way so that the forces on the crosshead help lift it against gravity, and reduce friction on the crosshead guides. For a vertical engine, it should not matter which way the engine runs, but there are valve considerations for vertical engines, and often asymetrical valves are used for vertical engines to counteract the forces of gravity, with unequal admission points on the upward and downward strokes.
Since the throw of the eccentric can vary from engine to engine, one way to figure out how much you need to rotate the eccentric in order to reverse the engine is by comparing the angle of rotating the eccentric between that required to begin admitting steam to both the top and bottom of the cylinder.
Edit 04:
Normally, on a full sized steam engine, you rotate the engine to TDC (assume this is a crank position of zero degrees), you then rotate the valve to the midpoint position (typically 90 degrees ahead of the crank zero position, then you continue to rotate the eccentric so that the valve is at the point of admitting steam to the top of the cylinder, which is generally another 55 degrees (thus the term 55 degree eccentrics), and then rotate the eccentric any additional degrees for admitting steam before TDC if desired). Dual eccentrics for a standard steam engine would need to be about 110 degrees apart in order for forward and reverse to work properly.
