I think the expansion you see in the bore at the boundaries between the body and foot sections is likely due
to back-reaming. A joint provides an opportunity to introduce a cavity in the bore and some makers take
advantage of that to tweak the bore profile. These cavities change the voice of the flute, and sometimes in
a good way.
I have come to this conclusion based on some experience making replicas of flutes with and without such
cavities. In one extreme case I had a flute with a longer socket than tenon at the foot joint. After disparaging
the original maker (privately) I set about making a new foot, with a longer tenon, to fix the problem, only to
find that it really spoiled the flute’s voice. I spent a ridiculous amount of time trying to address the problem,
thinking that I had some how messed up in my measurements. After making several new foot sections, and
trying various bore profiles in the foot, I eventually found that when I reintroduced the cavity, by cutting the
tenon shorter than the socket, the flute came back to life and played beautifully. I even made several inserts
to explore the effect of closing this cavity. I eventually concluded that the original maker really did know what
he was doing, and found a clever, hidden solution to a voicing problem. More often than not, I find that the
original flute make really did know what he/she was doing, so you have to be careful not to let your own
arrogance trip you up when trying to separate out the effects of degradation or trying to “fix” tuning anomalies.
I have also wondered if the cavities were the result of wear from tooling, specifically methods of work-holding,
or perhaps wear from years of cleaning out those sections, but they seem to be a bit too large for that
in the flutes where they occur and not present at all in other flutes. Also, they seem to occur repeatedly in
different instances of flutes from the same makers, so I think they are deliberate.
On the issue of why not just use cork and dispense with thread altogether, the problems I have encountered
with cork are mostly to do with the unreliability of the glue, over time. I have had corked flutes from several
different makers, all highly reputable, in which the cork has come unstuck. I’m not entirely sure what causes
the problem, but I suspect either interaction with bore oil and the glue, or perhaps use of a cork grease that
contains something that interacts badly with the glue. This may happen, for example, if someone uses a lip
balm that has oils added as a cork grease. So cork is also vulnerable to misuse by the end user, and a bit more
tricky to fix.
On antique flutes that are corked, the cork is nearly always deteriorated and breaks off in pieces. Since different
flutes all seem to have different clearances between tenon and socket, they all seem to need different thicknesses
of cork and it takes a bit of trial and error, plus a lot of sanding, to get a good fit. Threading ends up being a
simpler solution for dealing with such variability, and ends up being more reliable if done correctly.
If I can find a thread wrapping approach that is no worse than cork with respect to tenon compression, I would
prefer to use it. I have seen evidence that applying the wrong kind of thread the wrong way can cause serious
damage. I have yet to see compelling evidence that the right kind of thread, wrapped the right way, causes any
more damage than cork. Claire’s study shows equal deformation from cork and waxed silk. Terry’s studies show
an example of a flute with thread wrapped tenons that has survived over 100 years without ill effects.