The smaller the lower seat area of a conventional player valve,
the smaller the differential force necessary to overcome the suction
pulling the valve onto the seat.  For a given pouch diameter this
results in faster, more positive valve operation even at low suction
levels.

But reducing the diameter of a circular valve reduces the circumference
-- the peripheral length -- of its orifice, and since flow is, generally
speaking, a function of peripheral length multiplied by travel, its
travel would have to be increased to maintain the same flow.

However, too great a travel results in slower valve repetition,
significant suction losses around the valve while it is between seats,
unreliable soft playing and noisy operation.

Cross valves cleverly achieve a smaller orifice area than round valves,
yet a larger peripheral length, so a small travel can be retained,
resulting in fast repetition, positive soft playing and quiet operation.

In my experience, while 0.040" is a reasonable minimum travel for
circular Aeolian valves, 0.032" to 0.036" is good for cross valves,
depending on the size of the stack pneumatics.

Remember, while the applied suction level can compensate for some
constriction of the lower valve seat, the speed of opening of the
stack pneumatics is highly dependent on the flow through the top seat,
and thus on valve travel.

Where two or three cross valves are paralleled to operate the
sustaining pedal for speed with quietness, the travels can be much
less, and 0.028" sounds entirely plausible.

If the pedal regulator has to be tightened extremely to get the
sustaining pedal (and the soft pedal) to work smartly, the valve
travel is too small, and soft playing will become unreliable while the
pedals go on.  If the pedals bang, the valves chuff and the regulator
is backed right off, the travel is excessive.  Fortunately it's not
too difficult to remove the governor box and adjust the travel.

It's also worth checking that the muffling cloth or felt that Aeolian
fitted variously to the governors works without constricting the flow
or fouling the valve stems.

Patrick Handscombe
Wivenhoe, Essex, UK