In the process of rebuilding the 2-4-6-7 intensity valve blocks,
and other components of an Ampico "A" expression mechanism, I realized
there are some subtleties about the design(s) that I have never bothered
to "noodle through", and although my efforts have always turned out to
be functional, I am now wondering whether anything I am doing (or not
doing) should be modified.

As a start, it's useful to review some variants I have observed in the
design of the blocks, presented in _presumed_ chronological order:

Stoddard -- I'm not going to characterize Stoddard designs since the
ones I have seen do not have individual valves, rather they typically
have an integrated design with the 2, 4, and 6 valves all in one
combined module.  Some variants have the 7 valve in that same module and
some have it in a separate construction.

Variant 1 -- All four blocks (2, 4, 6, 7) have all four corners at
"full height" (see photo).  All 16 mounting screws are "long" at about
1-1/8".  There is a leather "donut" upper seat for the cancel poppet,
3/4" in diameter, glued to the cork block-to-deck gasket.  The top of
the cancel poppet is bare wood.

Variant 2 -- Same as Variant 1 except the top of the cancel poppet has
a very thin leather disk (pouch leather?) glued to it.

Variant 3 -- Same as Variant 1 except instead of the leather donut
there is a 3/4" felt donut glued to the gasket.

Variant 4 -- Same as Variant 3 except the top of the cancel poppet has
a very thin leather disk glued to it.

Variant 5 -- Three blocks (2, 4, 6) have their four corners "clipped"
by a 3/8" diameter mill bit which reduces the thickness of the block
in the milled area by approximately one-half (3/8") of the normal 3/4"
height of the block (see photo).  The 7 block has only the two "inboard"
corners clipped, the "outboard" corners are full height and use the
"long" (1-1/8" screws).  The clipped corners use "short" (7/8") screws
with metal washers.  There is a 3/4" felt donut glued to the gasket.
This donut is typically red felt, approximately 3/32" thick, and in
poor condition in an unrestored unit.  In some previously "rebuilt"
units, they are white felt.  The top of the cancel poppet is bare wood.

Variant 6 -- Same as Variant 5 except the top of the cancel poppet has
a very thin leather disk glued to it.

Variant 7 -- Same as Variant 5 or 6, i.e., no change in the valve
blocks (2, 4, 6, 7), but there is a thin, gray felt-lined, sheet metal
cover over all four blocks (see photo).  This has been seen only on
expression units that also have the sustain/loud pedal compensation
pneumatics mounted on the "spring" (expression) pneumatic.  This is
presumably for sound attenuation purposes, similar to the wooden cover
over the spill valve and amplifier pneumatic section of the pump.

Now for some assumptions:

1. At the point in time that Ampico engineers made a switch from
leather cancel poppet upper seats to felt upper seats, it is assumed
that this change was made to enable more reliable operation of the
lock & cancel intensity valve blocks.

2. Since variants with "clipped" corners appear with such late
refinements as the pedal compensation pneumatics and metal "muffler"
cover, they are assumed to be of more recent vintage than variants
without clipped corners.

3. Since all observed clipped corner variants also have felt donuts
as the upper seat for the cancel poppets, this configuration is assumed
to be preferable.  No clipped corner variants were observed to have
leather donut upper seats for the cancel poppets.

Discussion and questions:

1. In my "boneyard" I have numerous examples of the Ampico "A"
intensity valve blocks.  In "untouched", original samples I have found
a mix of Variants #1 and 2 within a single expression unit, i.e., some
cancel poppets with leather facings and some bare wood.

2. Likewise, some untouched original units have a mix of Variants #3
and 4 within the same unit.

3. Likewise for Variants #5 and 6.

4. In obviously non-original units, the same inconsistency occurs,
i.e., some poppets have leather upper faces, some are bare wood.

5. The bottom line is, there seems to be no consistency as to whether
the cancel poppets have leather upper faces or bare wood.

6. Intuitively, the felt upper cancel poppet seats would not provide as
good a seal as the leather seats, regardless of whether the poppet was
leather faced or bare wood.  But since this is a later refinement, why
would the engineers have made this change?  Was a slight amount of
leakage here found to be advantageous?  If so, why?

7. Regardless of donut/poppet mix, what should be the gap between the
two?

8. Is it possible that a leather covered poppet performs the same as
a bare wood poppet and that the leather cover (or absence thereof) is
simply the means by which the nominal gap is adjusted?

9. Shifting gears a bit, what are opinions as to the continuous pouch
leather (as shown in Variant #2) having the same "footprint" as the
valve block?  (Note: This was found on a partially rebuilt unit, not an
original.)

The thickness of leather used here (when only a circular disk) can be
critical; if it is too thick, there may not be good seals between the
block gasket and the various holes (and channels) in the deck.  I'm going
to go out on a limb and endorse this rebuilding technique as I believe
it will minimize block-to-deck sealing leakage.  The slight additional
leather consumption is insignificant when compared to reliability and
rework to correct a block-to-deck leakage problem.

10. Shifting now to the three cancel pouches on the underside of the
deck which inflate in unison, raising the three cancel poppets, in
response to a #7T/B perforation.  In essentially every original unit
I have observed, once the motor-weight cloth is removed from the bottom
of the deck in preparation for re-sealing these pouches, the "wells" of
these pouches are found covered with a layer or two of paper,
frequently newsprint.

I can think of only two reasons for these, either to prevent excess
glue applied during the replacement of this deck cover from dripping
into the pouch well and possibly fouling its operation or to prevent
or minimize any percussive sound associated with the three pouches
operating simultaneously.  What are the opinions on this conjecture
and opinions on the necessity for retaining these covers or the
advisability of omitting them?

John Grant

 [ http://www.mmdigest.com/Attachments/17/07/30/170730_215109_Variant_2.jpg 
 [ http://www.mmdigest.com/Attachments/17/07/30/170730_215109_Variant_5-6.jpg 
 [ http://www.mmdigest.com/Attachments/17/07/30/170730_215109_Variant_7.jpg