If neatly done, water-thin CA glue in small doses works wonders on
restoring tuning pin torque, with no downside I can think of.  Here
are the reasons I like it:

1. Low cost.  A two-ounce bottle is $10, and will often do a whole
piano.  Buy fresh from a busy hobby store.  Old glue won't do.  (You
need ultra skinny tips, too, so you can get the tip between the pin
and the bushing, or at least close.)

2. It heals rather than hurts the pinblock.  The old glycerin pin
tightener attracted water, temporarily swelled the wood, crushed it,
and eventually became even looser.  And the tuning pins rusted,
grinding the wood around them even more to dust.  Pounding in larger
pins will tend to make small cracks even larger.  CA glue seeps into
cracks and makes them stronger.

3. Feels good when tuning.  CA wicks in along the pin, filling mostly
the top quarter inch or so.  I've cut apart pinblocks to see.  This
restores the tuning hammer "feel" that a tuner likes.  Oversize pins
tend to grab most at the bottom of the hole, and skip, jump, and snap
as you try to tune.  I much prefer the feel of the CA glue job.

4. It does _not_ prevent repinning.  Driving a new pin into a hole with
a little CA glue doesn't present any problems.  If anything, the top
layers of wood are better able to hold together.

5. Does not risk breaking strings at the tuning pin, as removal and
replacement would if using a larger pin.

6. If it doesn't work (a rare event in my experience), give it another
treatment.  If that doesn't work (never in my experience) you can
always go to plan B (epoxy in the pin holes), plan C (larger pins),
plan D (pinblock inserts in uprights), plan E (complete pinblock
replacement).

7. Did I mention low cost?

If there are only a few loose pins, I routinely treat them during
a normal tuning visit.  If the piano is a basket case upright in need
of widespread treatment, it is better to use a piano tipper to put it
on it's back, so the CA glue is easier to place, and gravity helps the
capillary action.  But...I've done them standing up plenty of times;
it just takes longer to stay neat.  Grands need protection under the
pinblock to prevent CA glue drips on the action.

You will be pleased with the result.  I really have not seen any
downside, other than my competitors not getting pinblock replacement
jobs.

When not to do CA glue: If you are going to restring the piano, and
perhaps do soundboard and bridge work, put in a fresh pinblock and give
the piano another 50+ years of solid tuning.  But if the strings are
staying on, it sure seems safest to try CA glue first.

Caution:  Find a tech who has developed good CA technique.  Sloppy
application will quickly glue dampers to strings, lock up actions, etc.
You may want to open the windows and keep out pets if doing a big CA
glue job, as the ammonia-like fumes get strong.  Always have CA glue
debonder within arms reach, so you don't look like a fool glued to
a piano with the debonder a few feet away, just out of reach.  If you
have three thumbs, and don't really know the difference between a
hammer butt and a hole in the ground, let someone else do it.

Flame away -- my flame suit is CA-glued on!

Greg Graham, RPT