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Ordered a few bottom brackets; I've never seen this ring before. I can imagine its purpose, but that imagining takes me to consider that this could be problematic (bottom out before bolt is tight). So - what gives and is this new?enter image description here

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You mean the shoulder that is inboard from the square taper?

If the crank manages to snug onto the square taper far enough that it hits the shoulder but remains loose, then the crank arm is worn and needs replacing.

The tolerances will not allow a good crank to press hard against the shoulder, so don't worry.

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    I figured that's what it's for, but why is it necessary? Why is it only on one side? These are UN55 and UN26 bottom brackets. It's a recent addition and it's more work and more weight, so Shimano must have some purpose in adding it. Perhaps they have a new crank design that utilizes this? (And calling it a shoulder is interesting because the tapers have what I would call a shoulder. It's where the taper ends. This ring sits on the shoulder, it's like it's pressed onto the shoulder.) I'm curious what practical benefit the ring adds. – jqning Dec 2 '16 at 13:35
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    Could be some kind of lawsuit protection or regulatory compliance, where someone did something stupid, got hurt, sued, and now there's a thing to prevent it. It's on the drive side where there's more going on, suggesting it's chain/chainring related. – Jamie A Dec 2 '16 at 19:11

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