OK, I went down to the hardware store and got the Loctite. Dismantled the cranks and BB, cleaned up the threads, and did the job. I'll take it out for a spin in a couple hours, see how it goes. Did the best with what I have. I'm hoping it stays tight.

When I had it apart I saw a flat spot on the threads of the cup. About a 3mm diameter spot where the threads seem flattened. May be a cause of the loosening, I dont know.