Tining pin bushing purpose, was Xylol for tuning pin bushings

[Frank Emerson]

 > Mason & Hamlin said that one purpose of the bushings was to center the 
 > drill bit in the plate's hole, so that would be one downside.
 >
 > --Cy-- 

Other piano manufacturers besides M & H  will tell you that the primary, if
not the sole, purpose of tuning pin bushings is to center the drill in the
hole.  But, is that really optimal?  If not, is that really a "downside?" 
Given the angle of the drill (approx. 7 degrees) if the drill is centered
at the top of the bushing, it is considerably off-center at the bottom of
the bushing.  I would argue that it would be preferable to maximize the
bushing material on the side that will be compressed by the string tension,
which means that the drilling through the bushing needs to be deliberately
off center, opposing the pull of the string tension.  Look at any piano
with bushings, and you will likely see a gap between the tuning pin and its
bushing on the side opposite the string tension, and the bushing compressed
on the opposite side.  I cannot speak for Yamaha, but I suspect that this
is, at least in part, their reason for not driling a center hole, so a
center-punch can be used to mark the center for drilling, off-center,
opposing the pull of the string.  Using the bushings to center the drill
may be "good enough" to prevent direct contact between the tuning pin and
the plate, but deliberately off center towards the "top" of the hole would
be even better, IMO.

Years ago, I restrung an S&S grand (with no bushings).  Servicing it
regularly in a university environment, I had been irritated by several pins
"ticking" when I tuned it.  Every movement of the tuning pin, in either
direction, was accompanied by an audible "tick."  I marked the offending
pins before removing the strings and tuning pins.  After the pins were out,
I observed a visible impression of the tuning pin threads, on the inside of
the plate holes, in every tuning pin hole that had been marked as a
"ticker."  The thread-marks were always, and only, on the side of the hole
toward which the pin would be pulled by the string tension.  I took this as
clear evidence of direct, metal-to-metal, contact between the tuning pin
and the plate webbing.  I reamed out the tuning pin holes, to enlarge them
enough to add bushings.  This proved to me that tuning pin bushings serve
at least one purpose, to prevent direct metal-to-metal contact between the
tuning pin and the plate.

I have no experience with open-face pinblocks in a production piano, but I
suspect that there is a measureable gap between the tuning pin and its hole
in the pinblock on the side opposite the pull of its string tension.  While
it is clear that an open-face pinblock provides optimal support to the pin
higher on the pin, a tuning pin bushing provides similar support, when the
drilling optimizes the bushing material  on the effective side of the hole.


So, what is the purpose of the tuning pin bush?  I would say:

1. To prevent direct metal-to-metal contact between the tuning pin and
plate.
2. To center the pinblock drilling in production, or in some cases, to
deliberately off-center drilling, on the side opposite the pull of the
string.
3. To provide support to the tuning pin as high as possible, if not as well
as with an open-face pinblock, the next-best thing to it, 

Frank Emers