Square Tapered Bottom Bracket

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Square Tapered Bottom Bracket

Postby rifraf » Wed Feb 13, 2013 7:08 pm

Gotta buy a bottom bracket for new Surly frame (Ogre).
I have some Thorn cranks and having asked in the Thorn forum to identify the correct model and which size BB for a 54mm chainline I think I need a 113mm one.

http://www.thorncycles.co.uk/forums/ind ... 8.msg14315

Now Surly say " 73mm shell width, threaded standard English (1.37" x 24t)"

Now the question:
I've found two BB's which I think would do the job but am not 100% sure which is the superior one. I suspect its this:
http://www.jensonusa.com/Shimano-UN-73-Bottom-Bracket
But, being wrong often enough I thought I'd throw into the mix:
http://www.jensonusa.com/Shimano-UN55-Bottom-Bracket

So yeah, am I right thinking these are the correct "type" of BB needed?
Is the UN-73 superior like I suspect?
Or is the UN-55 the usurper to the role?
Or have I gone completely mad and am looking at completely the wrong type of BB for my square taper crankset?

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by BNA » Wed Feb 13, 2013 7:13 pm

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Re: Square Tapered Bottom Bracket

Postby RonK » Wed Feb 13, 2013 7:13 pm

I think the UN73 is the better quality part, but it doesn't matter much. I'm using a UN54...
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Re: Square Tapered Bottom Bracket

Postby TDC » Wed Feb 13, 2013 8:59 pm

The UN73 is the dura ace level one. The UN55 is an update of the UN54, with a metal non-drive side locking collar instead of the plastic one.

The UN55 works fine.
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Re: Square Tapered Bottom Bracket

Postby am50em » Wed Feb 13, 2013 9:10 pm

Just installed a Shimano BB-UN55 replacing the original Chin Haur CH51 Square Taper and it was trivial to install and is very smooth.
I measured the original one as 119.5mm so ordered the Shimano 118mm which is just a fraction short for the derailleur high/low settings.
A 122.5mm one arrived today so hopefully will get to install on weekend and get the shifting perfect.
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Re: Square Tapered Bottom Bracket

Postby Baalzamon » Wed Feb 13, 2013 9:27 pm

Bearer of bad news
The UN-73 is a 68mm BB won't work with your frame rifraf. Unless jenson have got it wrong on their picture, that is clearly showing a 68mm BB as it's stamped on it
Surly troll has a 73mm wide shell vs the standard 68mm. YEP Surly troll has an oversized BB.
One that Surly specs with the Troll is Shimano UN-55, 73 x 122mm. Hollow spindle, square taper, so go with the UN-55
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Re: Square Tapered Bottom Bracket

Postby rifraf » Wed Feb 13, 2013 9:58 pm

Thanks for all the replies
I'm now much better informed
Cheers :D
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