Removed a bottom bracket that was rough and rusted. Went to the bike Co-op and picked up a suitable replacement. I did not get a non-drive side cup since I had a couple at home.

I installed the bottom bracket, drive side first without issue. I could not get either of the NDS cups to thread in. I removed the bottom bracket and was able to thread in both NDS cups without any problem. Tried to slide the cups onto the bottom bracket and they will not slide on, they are too small, about ½ a mm. Tried the cup I had just removed and it will not fit either of the bottom brackets (“new” or old).

I’ve installed a few bottom brackets and never had this issue before. I thought these were interchangeable.

Anyone ever had this problem? I’m about to order a new one but thought I would post this first. Thank you.

I don't really understand all you are conveying but am aware of at least half a dozen differing threadings on cups and there are probably many more when you consider long obsolete cups, are you sure the ones you have match? Photos of the parts would help.

Why should one company offer an internal duplicate to another's? They sell complete BB units complete with the NDS cup that works with their bearing/DS cup unit. They see no need to match the internal aspects of the cup to anyone else's. Cheapest path is to go back to the coop with that BB in hand and see if you can match up a NDS cup they might have. Andy