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I'm installing a brand new BB-UN300 (68x122mm) into a 1991 Specialized Sequoia (replacing the pitted cup and cone BB). Out of the box, the BB doesn't spin very freely by hand, but that's to be expected out of brand new seals I guess.

The problem is when I fully tighten the BB into the drive side of the bike. That's when it barely turns at all, even with the crank arm attached. It's hard to explain how hard it feels to spin, so I'll just explain it like this: crank arm attached, moved up 90 degrees, released, and it goes straight down and stays there, without moving from side to side at all.

Any idea why this might be happening? I would understand it if it happened when tightening the NDS, since that would put excessive preload on the bearings on that side, but this happens with just the DS threaded in. Why would the torque on the DS affect how freely the spindle spins?

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  • It’s okay for the crank to not spin like crazy. Perhaps your seals are just slightly tighter than spec. As long as you don’t feel any noticeable grinding, you’re all good.
    – MaplePanda
    Feb 1, 2021 at 2:41
  • @MaplePanda sure, that's why I mentioned the seals myself in the beginning of the question. But the question is, why does torque on the BB affect the spin at all? Feb 1, 2021 at 6:43
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    "crank arm attached, moved up 90 degrees, released, and it goes straight down and stays there, without moving from side to side at all.". That sounds about right. Put both cranks on, but no chain. Hold the seat tube by the BB shell and spin the cranks. If you feel no grinding, you should be good to go. Feb 1, 2021 at 6:53
  • Did you end up with any conclusion? If the threads are not aligned, they can be corrected in a bike shop by a tap. Sep 20, 2023 at 12:17

2 Answers 2

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It is possible for BB shell distortion from welding to cause excess friction in BB cartridge units. Usually this is accompanied by roughness screwing in one or both parts of the cartridge. In those cases, chasing and facing the shell typically eliminates the issues, and there's not much else that can be done.

What you describe could be the normal friction of a new lower-end cartridge, which will tend to get better in use, or it could be a symptom of the above. Doing a test with a crank as you describe may make it harder to see what's going on, because the crank creates so much leverage. I would install the cartridge in the normal way and then test how freely the spindle turns by hand. A little friction is okay but it shouldn't be much more than that.

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  • Hmm, the BB was indeed a bit difficult to screw in. I haven't cross-threaded it, of that I am sure, but there were "sections" of it that didn't go in the shell very smooth. I did turn it by hand too, of course, that's when the problem is even more palpable. Feb 1, 2021 at 6:46
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I had a very similar experience with my BB UN300 68x122. I inserted the drive side, then the non drive side for a few turns only (i.e, it was still sticking way out of the frame). Then I went back to tightening down the drive side and when satisfied with the torque applied (by feel), the spindle was barely able to rotate. I did not use a torque wrench (did OP use one?) but I do not think I went above the 50-70nM spec. The insertion/tightening was smooth, no cross threading or other issues. Unscrewing the partially inserted non drive side went smooth and changed nothing.

Still, after loosening the drive side a half a turn the spindle rotated reasonably well again (not as freely as when not installed, but not grinding either) and the unit was still pretty tight to the frame. This has made me question if the cause was indeed over-torquing the drive side. I installed the NDS and have done 400 km with it since with no issues so far. Perhaps I have damaged the bearings/races from over torquing, reducing the durability.

Exactly how this happens I do not know. I very much doubt having the NDS in a few turns at installation caused this. (Shimano does not tell us to do this, I picked it up at some forum. Perhaps I should have just listened to Shimano instead)

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    I believe installing the NDS before torquing the DS is fairly common (I do it), as it helps a bit in ensuring that the NDS threads on cleanly. In fact, the general practice for many critical and non-critical assemblies using multiple threaded fasteners involves loosely threading on all fasteners prior to torquing them. Additionally torquing can even spec’d to be done in stages and/or in a specific fastener sequence (e.g., cylinder heads in an automotive engine).
    – Ted Hohl
    Sep 20, 2023 at 14:57

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