Hi All,  Skip Downing asks about the 'Backwards Test' and the
Lauter-Humana air motor.  The motor should pass the test with at least
an "A-minus" grade.  ("A" is very good, and "A-plus" is excellent.)

The sliding valves should not be snug.  If anything, they should be
a tad loose fitting in the guides, both up and down and side to side.
This allows them to 'find' their seat.

The Lauter-Humana air motor has a unique sliding valve that is
comprised of two separate pieces of wood.  For reasons that have never
been completely clear to me, they took a rectangular box and cut it
in half on the long side.  Then they glued pieces of leather between
the cuts, making the valve somewhat 'flexible'.  Having those pieces of
flexible material between the two sections of the slider makes it very
difficult to re-surface the slider.  To overcome this problem, I use
a piece of hardwood (like the dip block used to level keys) to keep the
top of the slider flat.

Furthermore, it is because of the pieces of leather that the motor
cannot be as "tight" as a motor with 'solid' sliders.  Ergo, it can
never be an "A+" quality motor.

Also, re-surfacing the block is equally challenging because the surface
is only slightly larger than the slider itself, and it's raised off the
body of the block.  (Actually, it's a different piece of wood that's
glued onto the block.)  However, because of the way it's designed, it
is seldom 'out of flat' by more than 1/1000th of an inch.  So, it's
wise to check it carefully with an extremely flat edge and very bright
light before making the decision to make a change.

Lastly, like virtually every original air motor I've encountered, the
main vacuum supply channel in the Lauter-Humana air motor always leaks.
In fact, it seems to leak more than average.

I believe there are two main reasons for the leakage.  One, the wood
that Lauter used is relatively soft (see note below) and, as it has
been discussed previously in this forum, all wood leaks as it ages.
The second reason is that the motor is constructed of three sections;
between each section is a leather gasket.  And, unlike the old Aeolian
air motor which was similarly constructed, there is no facility for
compressing the gaskets.

The Aeolian air motor has two long threaded rods which are used to
compress the rubber gaskets between the three sections.  In the Lauter
motor, the three section are held in position by two pieces of wood
that are screwed onto all three sections.  And, even if the leather is
replaced during the rebuild, air can still leak through the leather
because it's not compressed like a normal gasket.

To seal the main vacuum channel 100%, I always use Phenoseal.  However,
the procedure I use with the Lauter-Humana air motor is slightly
different than normal.  Normally, I just seal the ports where the
sliders go, pour in the Phenoseal, slosh it around, and pour it back
out.  With the Lauter motor, I stand the motor up vertically (using
bricks to hold it in position), pour in the sealer until the channel is
full, and then I leave it there for about five minutes.  My reason for
sealing it in this manner is that I want the sealer to permeate the
leather and the soft wood.

After sealing the channel, I also seal the pouch wells and air ports
leading to the wells.  These areas were originally sealed with shellac,
and although Lauter did a very good job sealing them, my thought is
"an ounce of prevention is worth a pound of cure".  Also, since the
valve block is glued to the main block, there's always the possibility
of leakage.  Also, as a precautionary measure, I also seal the inside
of the sliders.  This helps reduce the vacuum loss through the leather
pieces that connect the two halves of the slider together.

Lastly, and addressing Skip's concern that the Lauter air motor cannot
achieve a passing grade on the 'backwards test' because of its design.
If that were true, no motor with counter-opposing bellows could pass
the test.  Fact is, the basic principles of operation of the
Lauter-Humana air motor are no different than any other air motor.

(Note: In my opinion, Lauter elected to use a softer-than-normal wood
to make it easier to bore out the pouch wells.  As those who have seen
the wells will agree, the inside surface is very rough, and I believe
it is for this reason that Lauter used copious amounts of burnt shellac
to seal the wells.)

Musically,
John A Tuttle
Player-Care.com
Brick, New Jersey, USA  (40 minutes from Newark, NJ. where the
Lauter-Humana was manufactured.)