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The cups have opposing thread directions to ensure they tighten during use rather than loosen. So you may end up with it torquing itself, especially with binding bearings. I would also see if you could have the BB shell chased and faced. Cruddy threads or out of parallel shell faces could cause the cups to go out of alignment at higher torque. But a UN55 might be cheaper.
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You've not bought a 73mm shell version by any chance? The UN300s don't seem quite as nice as the 55s but I've never seen one do that, they're still relatively decent parts.
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This was exactly my experience with a Campag Centaur BB (in a brand new bare metal frame). I removed and refitted a couple of times but couldn't see any reason for it binding other than poor tolerances. The manual even says something about backing off the torque and using loctite if it binds which I felt was a poor solution.
Even at the reduced setting it was still too tight IMO.
Sent it back to the supplier in the end who claimed there was nothing wrong with it.
I then read about a couple of other people who had exactly the same issue!
Switched to a Sugino and it was perfect.
Can you return the BB you have? at least to see if another of the same model behaves the same...
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@nejib
I have a square taper bb that binds up when I tighten up the nds cup/sleeve. It spins very freely when the cup is in there finger tight but when I tighten it fully there is a lot of resistance in the spindle (noticeable with the cranks on as well). As I understand this could be due to the poor manufacturing tolerances of the UN300 compared to the UN55 and bearings not being perfectly aligned...?
Would it be best to run the nds torqued up and put up with the resistance or run the nds a bit loose? Any downsides to the latter?