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New BB doesn't want to fit properly...

by windyweymouth
Tue, Mar 29, 2016 11:01
4

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windyweymouth | 8 years ago
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@srchar

That's my thoughts now, since the UN71 fits perfectly and the UN55 doesnt, I'm feeling inclined to return to Wiggle.

But a question on the UN55 version, both cups have a rim that when the BB is out of the frame and the cups held tight on the body, measures about the 68mm between the inner faces of the rims. 

So fitting the BB I'd assume the cups should screw in snug to the frame? 

BTW UN71's are discontinued by Shimano, all that's available is the UN55, which is an inferior bearing.

I guess I could opt for a Tange or something. Has to be JIS though.

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srchar | 8 years ago
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I helped a friend with a similar issue and it turned out that the BB shell threads were not in alignment.

However, given that your old UN71 runs smoothly, my instinct is that there's a manufacturing defect in the new part. Return it to the retailer for exchange and have another go with a different UN71.

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windyweymouth | 8 years ago
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I couldnt be certain about the old spindle, it certainly feels very buttery still, in or out of frame. The new BB is smooth out of the frame, but only starts to bind when the cupps are torqued in. I can return it to smoothness but that means the left cup is about a mm or two away from the frame.

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. . | 8 years ago
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Quote:

I wanted to replace my old Shimano UN71 because when I took it out the cups got a bit chewed in the process and felt that maybe better start afresh with new BB and new cups. So maybe I'm doing something wrong here, put in drive side first, tightened that down. Put in other cup, screwed it in and it became progressively tighter, thought nothing of that. But with the cup screwed in as far as it will go, the bearings are no longer smooth to turn. Threads are not crossed. Put back the old BB and even after making everything really tight, spindle runs smooth. What's up? Is the new BB not as accurate as the old BB? Size is correct, 68mm on the frame and 68mm between cup inner faces.  Maybe designs have changed and one is not supposed to screw the cups to touch the frame....certainly on the UN71, the free cup has no rim and can be screwed in to feel tight. But even when screwed in tight, the spindle runs smooth

 

Is the spindle straight?  The old bearings may have worn to accommodate a bent spindle.

Do the bearings spin smoothly when not inserted into the frame?   How about when just one side is inserted?  Or does the problem only start when you torque them up?

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