I have a 1980s Holdsworth bike and just recently bought a new 3 piece bottom bracket set, as the original one was pitted on the bearing surfaces. I fitted the new cups but as soon as I put any pressure on the pedals the adjustable cup popped out. On investigation I found the threads on the frame had rusted and the threads had lost their profile. The strange thing is that the old cup doesn't pop out under peddle pressure and at the moment I am using it (reluctantly).
Has anyone got any ideas of what I can do to solve the problem.
I'm on a tight budget by the way.