Alternator not charging on my 140

Everything is rebuilt or new. Nippon Denso alternator stopped charging the battery.
I looked around and found a burned rubber cover on the end of the engine bay starter solenoid. The locking nut is very rusty!? And fully tight.
What might have happened, and what is the remedy?

Thoughts: Maybe the wire from the alt to the starter switch is too small for the amperage? The original wiring only carries about 40 amps max. I connect my 65 amp Lucas directly to the battery through a marine 80amp fuse.

A rusty connecting nut is a sure sign that the stud is corroded, and not very much amperage will pass through it. An alternator may try to push it through anyway and end up with a lot of heat in the wire and something will burn out.

Thanks Rob. I wonder why the stud and nut and washer are so rusty. I have now sprayed thin, penetrting oil on them, as the nut was rock fast.
Brush off the rest, apply electro thin oil, and see if things improve?
The cables have the same thickness as on 2 other XKs.
Replace thevstarter solenoid?

Keep working on it gently to get the nut loose, penetrating oil every day, very low torque on your socket. Too much torque and the stud will break away inside. Sometimes they can be saved. Better figure on a new solenoid though anyway.

After 2 days with penetrating oil, the nut is loose. I used the digital thermometer on the nut, with engine idling, but there is no signs of high temperature on the nut or cable, yet the rubber cap from cable to nut og the solenoid stud had melted at some stage, prior to the alternator/battery not charging issue.
I still only get 12,5 off the battery poles, at idling, with the Nippon Denso alternator, at 600 rpm idle , so there is a problem.