The threaded rod is
Du-Bro #379 4-40; their website notes that it's made of stainless steel. The two push rods are
Du-Bro #145 4-40 but the metal is not specified.
I've alternated between two types of solder and two types of flux with similar results.
One type of solder is Kester 60/40 Rosin core - the solder is fairly thick and the spool is probably at least 20 years old (leftmost in picture). The other spool is also Kester; the solder is much finer but the content is not specified (one from left).
The flux I used is for Lead-Free solder, which in retrospect seems like a bad idea - 60/40 solder is part lead, right (one from right)? I did try a bit of another type of flux as an experiment and it seemed a little better (rightmost).
The end result is very strong, but it looks like $#!+.
After reading a lot of replies on various forums I've identified some possible trouble spots:
- Rods not clean enough
- Copper wire not cleaned
- Wrong solder type
- Wrong flux type
- Too little flux
- Too much flux