Clinton Gray wrote in 111209 MMDigest:

> ... the piano's airmotor was a little hesitant to move the roll.
> When set at Tempo 130, the roll played at about Tempo 70-90, and
> would slow and then go quicker, etc.  How is that fixed?

Beware of lengthy and boring commentary that follows.

Clinton, as far as the air motor, it sounds like it needs a standard
rebuild.  The Tempo issue is probably due to the slider valves are
not sealing properly.  The hesitant issue could be three things:
(a) the cloth of the pneumatics has taken a set (hard), (b) the hinge
is glue bound, or (c) there is a leak in the cloth.

A check for air tightness is done by putting your finger over the
exhaust nipple and turning the motor crank backwards.  On a well
rebuilt motor it should be very hard to turn backwards and the
restriction should be the same for the entire rotation of the crank
shaft.  Your motor will surely fail this test.

The first thing to do is remove the chain from the motor and rotate the
crank about 20 degrees and let it go.  It should not move backwards.
Repeat this process again until you go through one entire revolution of
the crank.  If it ever moves backwards, then the cloth needs replacing.

The next test is to check if the cloth is air tight on each of the five
pneumatics.  Remove the slider valves and any hardware on the main
trunk of the motor that the valves slide on.  Label all parts so they
can be put back in the same spot they were removed.  I always do this
labeling in an area that won't be seen when the motor is reassembled.

What you are going to do is close off the bottom hole going to the
pneumatic and turn the crank to check for leaks.  Turn the crank so
that the pneumatic you are checking is closed.  Now put your finger
over the lower hole and turn the crank so the pneumatic under test
opens up.  If it is hard to turn, then the cloth is air tight and OK.

Repeat this process on all of the pneumatics.  If the cloth on all five
pneumatics doesn't leak much, then you may be able to get the motor to
work by just lapping the sliding surface, valves and lubricating.

To fix leaky sliding valves you will be hand sanding the wood on a very
flat surface.  I use a thick piece of glass but you could probably use
a good flat counter top.  I start with 80 grit to get the valve
surfaces flat then go to 120 grit then 400 grit for the final sanding.

Place a full sheet of 80 grit sand paper on the flat surface and start
sanding the large valve surface of the main trunk with the grain of the
wood.  Try to keep the sandpaper from sliding or bunching up on the
flat surface.  You don't want to take much wood off, just make it flat.
Check every so often and see if all the old graphite is sanded off.

When you get to this point, use a medal ruler and check for a perfectly
flat surface.  Repeat if not flat.  Do the same procedure to each of
the sliding valves.  When every thing is flat, switch to the 120 grit
and sand some more until smooth.  Repeat using 400 grit which should be
smooth enough.

Blow off any wood dust and rotate the shaft quickly to blow out any
dust from the pneumatics.  Now lubricate the wood using graphite and
isopropyl rubbing alcohol (the 'First Aid' type).  Moisten the wood
using the hard felt of an old piano hammer dipped in alcohol.  Place
some graphite on the wood and rub it in using the piano hammer.  Repeat
until the entire sliding surface is covered with a thin layer of
graphite.  Also do this to the sliding valves.

Reassemble the motor, making sure that the sliding valves don't bind
between the guide strips -- in fact, the looser the better.  They may
seem sloppy but when a vacuum is put on the motor, the valves are
sucked tight to the wood and they seal.  If everything is airtight, it
should be difficult to turn the crank backwards with the exhaust line
plugged with your finger.

Install the motor and see how it works.  If the motor is still jerking
as it turns, you may have to time the values.

Pete Knobloch
Tempe, Arizona