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My MTB has a 73mm bottom bracket shell but it had 68mm BB installed. I found and purchased a Shimano BB-ES300 73mm BB but unfortunately I had to break apart its plastic cup (left side) since I misaligned it when installing and it got stuck in the shell. I wonder if a 68mm left side cup is exactly the same as a 73mm cup and they are interchangeable. If so I'll look to find a 68mm cup as they are more abundant.

I also wonder if installing a 68mm BB on a 73mm shell would cause cross threading. Because the BB shell of my bike is cross threaded (and I cannot fix it).

According to the Sheldon Brown's website they have a little bit different threading as 68mm is 1.370" X 24 tpi and 73mm is 1.375" X 24 tpi.

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The cups without a lip are interchangeable on the non drive side. If you use a 68 cup on a 73, you risk the lip bottoming out on the frame (in some circumstances) before the intended contact is made with the main cartridge.

Threading is identical, the same tooling is used to cut the threads for both 68 and 73 shells.

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  • Thanks a lot. Any cup without a lip will do the job as I understand.
    – Ender
    Commented Apr 25 at 19:48

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