I just had a leaking cross valve on my upright Steinway Duo-Art and
had to remove some seal cloth to coax it back into operation.  This
made me start to think again about possible ways to prevent rotation
of the valves, especially since the old tough stake-tanned leather is
no longer available.

Why not just press a little pin through the top valve leather and
into the valve body in the area of one of the slots in the valve plate?
A tiny pilot hole might be needed.  If the pin sticks above the valve
plate in the closed position it should limit the rotation.  If there is
concern that the pin will wear a slot in the valve plate, a short
bushing of Teflon could be held in place by a pin and this would never
stick or cause wear.

Any opinions on whether this might be worth the try?

Spencer Chase
http://nt.mcn.org/spencer/