So I'm about to make some new stronger pannier rails & a rear rack for the 640, will use 5/8 4130 tubing. First thing will be to machine up some mounting bushes from solid bar.
Now, I'm probably overthinking this but...
the local steel store has 4140 I could use (& the few scraps needed won't cost much), or I have some plain mild steel on hand. Am I likely to get an appreciably stronger joint with the 4140 to 4130 (will TIG weld & post heat treat with the MAP torch if required) or is there not much in it?