I have an old frame (60's vintage), into which I'm trying to install a modern sealed cartridge bottom bracket.
I've got a bog-standard Shimano BB-UN55 with diameter 68mm, which should fit the frame. Indeed, if you hold the cartridge up against the bottom of the shell on the frame, it's definitely the right size (phew).
However, I can't actually screw in the cartridge to install it. It turns out this is because there is a sort of wrapped around metal sheet inside the shell, which starts about 12mm (1/2"?) in on each side and the Shimano cartridge only gets narrow in the middle about 17mm from each end.
I assume this sheet is just to stop grot falling into the non-sealed bottom bracket from the frame tubes and I can probably pry it out with a screwdriver or similar, but wanted to check: should this be there for some other reason?