On Tuesday, December 12, 2017 at 12:32:54 PM UTC-5, N_Cook wrote:

What scarf angle do you use for the join?
nitrile/neoprene rubber cord ?

Neoprene.
And as sharp an angle as I can manage with a fresh X-Acto blade and clean mating surfaces. I would guess around 30 degrees or so. I have a marked, slotted block so that I can set the O-ring in the block and stretch it a specific amount so each cut is the same angle. I make a sloppy joint (extra glue), then more glue, dip it in baking soda (accelerant), allow to cure, then file the flash with a jeweler's file. It almost takes longer to describe than to do it.

Here is a picture from Phil's Old Radios:

https://antiqueradio.org/art/Zenith12S471NewBeltCut.jpg

Nothing wrong with a single-edge razor.

Peter Wieck
Melrose Park, PA