On a lot of earlier bikes with the wiring continuing through a hole/holes in the headlight, there were usually rubber "donuts" in the holes, to stop chafing and wire insulation from rubbing off. These 'insulators" will always rubber rot, fall off, get cut, compromised in some way, and then problems can get worse in one quick hurry. The actual holes in the metal have the edges rounded off, so, in effect, the wires shouldn't chafe and fail, but, they do.
I completely agree on circuit breakers, and I try to use them as much as possible on all my vehicle projects, motorcycle, car, truck, whatever.
One thing I have seen on a lot of our earlier triples and R/RD bikes is that early rear brake light switches can lose the plastic insulator on the pull piston, shorting the feed wire to direct ground when the rear brake was applied. The volt path would be through the feed wire, to the contact terminal, to ground through the brake light switch over-pull spring. Of course, the fuse would blow ASAP. Those switches were easily disassembled, and new insulators could be fabricated very simply, so, the switches could be rebuilt, with better insulating bushings. When I did this in the beginning, I also added an inline fuse and holder in the rear brake light feed wire, so if the switch had another grounding issue, it wouldn't kill the entire bike's power.
These early switches had threaded mounts/adjusters that were metal, with an insulator disk on each side of the holder nuts, to keep the switch from grounding on its threads in the frame boss. Later switches were all plastic on their mounting/adjustment threads. The later plastic switches could still fail if the pull-stud insulator inside them was compromised, too, through the pull spring to brake lever ground.
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