Hi,  I assume the original query applies to a dislodged nipple.
I have done a few of these over the years; it's not a task that should
be undertaken by anyone who has little or no soldering experience.

However, having said that, this is how I tackled the problem of a broken
nipple from the middle of a tracker bar.  First, clean the area of old
solder using fine chisels, files and/or scalpel blades -- anything will
do as long as it gets in and does the job.  It's not necessary to clean
back to bare brass; usually bright solder will take fresh solder when
applied.

Most tracker bars have a small drilled recess in which the nipples
would be "jig" mounted so, if you can get back to this, it will help
tremendously.  Check that the nipple fits back into the newly prepared
position correctly.  Then take a wide-mouth spring clamp and hold the
centre section of the tracker, placing a thick rubber pad on the side
with all the nipples so as to support and keep them all in place
including the loose one to be soldered.  On the other side, leave the
metal clamp to bare on the face of the tracker to give a bit of heat
dissipation.

This is the clever bit.  Apply flux to the area you want to solder.
Then, using a small butane torch play it up and down the length of the
tracker to warm it overall.  Gradually reduce the strokes so that you
apply the heat directly to the metal of the tracker and not the nipple.

(Generally, heating the length of the tracker will help to limit warping.
Applying heat to the nipple itself will cause it to heat and burn, thus
making it impossible to take solder and you will have to start again.)

This is where the skill bit comes in, for as the temperature rises and
transfers to the nipple you will see, very briefly, the solder in the
area "give" just for a split second as it becomes "plastic".  (Anyone
who has done lead loading on a car body will know what I mean.)

Start to apply the solder and by the time you touch it to the base of
the nipple it should run readily.  Immediately as this happens remove
the heat source and allow the whole thing to cool slowly.  You should
have a neatly soldered repair.  If solder has run into the hole use a
small drill or drills to clean it out, or plug the hole with a cocktail
stick before you start.

There is another way: glue the nipple in with epoxy -- you don't burn
your fingers and you don't need the skill of being able to solder.
Epoxy also works for broken reed resonators on theatre organs, just
drill the reed block and glue the resonator back in; easy, job done.
I know of one Compton and one Wurlitzer 260 where this was the prefered
method of repair.  God, I'm going to get lynched for that.  Bye Bye!

Paul Camps