My upright Duo-Art has 3-7/8" as the regulator span. I don't believe
this is a critical dimension. It might be typically different on
grands. I think that Bernt Damm was asking about dynamic valve gap
whereas Bill Chapman was referring to the stack valve gap.
The dynamic valves are fed by the dynamic valve and pedal regulator so
the required gap has nothing to do with the "0" setting of the piano.
There are varying opinions as to the ideal gap and also whether they
should all be the same. Some people say that the smaller accordions
should have a smaller valve gap and the larger should have a larger
gap. This does not make sense to me. If Aeolian intended to regulate
the timing of the accordions to an exact degree, it would make sense to
use an orifice to throttle the vacuum, not a valve gap setting which
could vary over time and would be impossible to adjust.
The time needed to open dynamic valves is a small part of the valve &
pneumatic operation cycle so there is little point in using a smaller
gap to speed up the operation of the smaller pneumatics. A small
amount of over-extension of the small pneumatic valves is of virtually
My advice is to use a rather large gap, .045" or greater, for all
dynamic valves. You really want fast but quiet operation, especially
of the pedal pneumatics, so you might want to set the vacuum level
pretty low and you will not be able to do this if the dynamic valve
gaps are too small.
Also consider that the dynamic valves are very easy to get to in case
you change your mind.