Hello, MMD.  Recently, I began to feel that my Steck Half D-A wasn't
playing Themodist rolls properly, so I made a second manometer to
measure Theme and Accompaniment vacuum levels simultaneously.  On
discovering that the two were essentially identical, I pulled out
the expression box.

Nothing was wrong with the interior; the problem was the locking
screw on the shaft which connects the expression knife valve with the
outside of the box: the screw was slipping on the shaft.  The chopped
out appearance of the locking screw told me that this had been
a problem in the past.

I have two questions:

(1) Is the correct thread for a replacement screw American 6 gauge
UNC (32 threads per inch)?

(2) Why on earth did Aeolian equip their full Duo-Art expression
boxes with both a locking screw and a set screw on each of the pair
of control shafts, but only one screw on the single control shaft
of a Half Duo-Art?

John Phillips in Hobart, Tasmania