I just finished rebuilding a Duo-Art expression box on which there
was a very clever modification that I would like to describe.

The clamp screws for the knife valve setting levers were originally
just slotted screws threaded into the split iron lever.  This is not
the best way to clamp the lever on the shaft, especially as the threads
wear over time.

This is a difficult part to duplicate if it should become stripped, and
worse is the case where the shaft is not clamped adequately and the
lever rocks, allowing the adjustment worm to eat away the spur gear cut
into the shaft.  This is impossible to duplicate without a gear cutting
arrangement.

On the upright, with which I have the greatest familiarity, I always
replace the screw with a stainless or hardened socket head cap screw
so that it can be tightened more securely with an hex key socket on the
end of a flexible driver, which allows more easy access in the tight
confines of the upright installation.  This can be an improvement on
the grand where access is good but the original screw is inadequate to
clamp securely.  The technique found on this latest box is much better,
however.  Unfortunately, it might not work on the upright.  I intend to
try it the next time I take the box out of my piano or on the next
piano.

Instead of screwing a screw into the threaded lever, the solution is to
thread the screw in from the opposite side and cement it in place with
Loctite.  It can always be removed later with a little heating, if
necessary, but this repair is likely to outlast the box if a good screw
is used.

Pick a screw of sufficient length that a nut and lock-washer can be
threaded on the free end.  Now you use a nut driver to loosen and
tighten the clamp.  This will work even with levers whose threads are
almost stripped.  If they are stripped you can solder the screw in
place or use epoxy or thread repair cement.  All it has to do is to
keep the screw from turning when you loosen the nut.  The screw head
and the nut do the clamping work instead of the weak old threads.

I would do one of these modifications to all boxes even if the threads
are still fine.  This will insure that the connection is always tight
and that the old parts continue to last.  I have seen several Duo-Arts
that did not play at low expression levels because these clamp screws
were not tight.  They can be very difficult to tighten if the parts are
worn and of the original design.

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