bottom bracket on the move

[Berlinbybike]

My Miche Primato bottom bracket constantly moves to the right - every 100 miles it shifts within the braket by about 3mm. The odd thing is that the left cup moves with it and so it remains tight. This, I suppose, is partly due to the threading in the BB (BSA) and the lack of lockrings on the cups. Anybody seen this, or similar? The cure (other than some other BB!)

 

[rusty bearing]

I see what you mean when I looked it up. Initially try some loctite on the threads when assembling.