Brand new Shimano BB-UN300 hard to spin when drive side is fully torqued

Question

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?

Nathan Knutson · Accepted Answer

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.

Brand new Shimano BB-UN300 hard to spin when drive side is fully torqued

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