Pretinned and "bunch tinned" as we used to call it are different; the
pretinned has each strand tinned. Bunch tinned has the group of
strands "glued" together with tinning or solder. Which is Tefzel
aircraft wire?

Good question to which I cannot find a definative answer on the web.
However, the Tefzel wire which I have worked with, definately would be
called "bunch tinned" (just this side of solid wire). I have always
felt that the importance of this is so that if the wire covering
becomes abrated that individual strands of wire are less likely to
fray and become a [short/spark/fire] hazard.

As was mentioned, "bunch" tinning causes the wire to become stiffer
than untinned. It hadn't occured to me until just now that this is
another safety attribute as the stiffness mitigates vibration of the
wire and thus reduces the possibility of abrasion.

Note that Tefzel refers to the cover of the wire (not the metal wire
itself) so I suspect that some "Tefzel Wire" may not be aviation
worthy. Look for the MIL specification to be sure. MIL-W-5086A for
non-shielded and MIL-C-27500 & replaces MIL-C-7078/3 for shielded.

The important of Tefzel is that 1) is very smooth and slick and
doesn't abrate easily and 2) when burned doesn't emit toxic gasses as
PVC (the typical wire covering) can.

Finally, I understand everyone's comments about "What is the big deal
about the type of wire we use?". We don't have fuels, we don't have a
lot of vibration, we don't typically have lots of sharp aluminum
edges, etc, etc. However we can get (and I have seen) gliders with
shorts and smoke in the cockpit. The amount of cost difference
between "crap" wire and Tefzel wire is just too small to be a reason
to avoid it. The real reasons are; that many people don't know know
any better and that it is just easier to buy the cheap stuff.
Hopefully, I have blown holes in both arguments.

Bite the bullet and do the right thing!