Mike Carey was asking about the felt stuffed in the expression box of his
Duo-Art. He should replace that with 1/4" thick muffler felt (made by
taking 3/8" or 1/2" thick felt and splitting or separating it). It works
best to taper it with a felt knife, roll it around a 1/4" drill shank,
and stuff it, overlapped a little at the tapered end, into the hole.
The felt is supposed to project out of the hole about 3/8" and kind-of
"flare" open a little. The center hole is supposed to be 1/4" ID. This
is very important to the linearity of the box. Any modification to this
muffler makes a very large difference in the expression "curve."
Note also: The 1925 Duo-Art manual shows a very strange-looking muffler
and I doubt that it would work that way. The picture was of a new,
(most-likely) unregulated box taken by the drafting department, so I
suspect the felt was still loose at the time.
I suggest not gluing it in, either. When you get it trimmed right and
tapered, it will stay where you put it, but if necessary, you can "adjust
it" as long as you don't glue it. Be sure to remove ALL of the white
fuzzy stuff. The correct felt is available from supply houses. It's
called "muffler" or "shutter" felt. It is gray in color.
Regarding crash valves, some Duo-Arts had separate crash valves mounted
above and to the accompaniment side of the box, and some pianos didn't
have one at all. If you wish, you can add one without much trouble, as
long as you are going to break into the box anyway.