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Using 73mm bottom bracket on 68mm shell to increase chainline

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Using 73mm bottom bracket on 68mm shell to increase chainline

Old 07-04-22, 04:09 PM
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milesfg
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Using 73mm bottom bracket on 68mm shell to increase chainline

I am fixing up an old Diamondback Ascent that has a Sugino GT triple that I would like to keep using. The cup and cone bottom bracket was very pitted and damaged; I don't think it is usable. The spindle is rusty and not in great shape either.


Old BB spindle is asymmetrical and 127mm (40mm driveside + 52mm center + 35 non-driveside). I replaced it with a Shimano UN300 68mm x 127mm BB, because that's the longest they seem to offer. I got the cranks on and there is maaaybe 1mm clearance between the granny ring and chainstay, and the chainline from the middle chainring is around 42mm which is obviously not ideal.


If I use a 73mm x 127mm bb bracket (Sunlite makes one) on my 68mm shell, will that give me a few extra mm clearance and a better chainline? Would I need a spacer? Or can I use a spacer with my current 68mm UN300? Any other options besides an expensive Phil Wood BB?


Thanks in advance,

Miles
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Old 07-04-22, 04:18 PM
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1mm of chainring clearance would be enough for me. A bottom bracket intended for a 73mm shell would actually take you in the wrong direction without spacers. In your shoes, I'd just use a 1-2mm spacer under the DS cup of what you've got and call it a day.

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Old 07-04-22, 05:28 PM
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As the above post states, it is enough, it clears. However, I had that happen with 6500 crank set that I installed on a Poprad. I just got a bb spacer from Wheels Manufacturing, 1.8mm thick, put that between the shell rim and the outside lip of the drive side cup, all was good. IIRC, the clearance before the spacer was only 1 mm. I was not comfortable with that, even though others said it was fine.
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Old 07-04-22, 06:05 PM
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Thank you both! I've ordered a spacer and I'll give that a whirl.
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Old 07-04-22, 10:37 PM
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I had a bike that had about 2mm of chainring clearance, and the frame flexed enough under heavy pedaling that the teeth scratched through the paint on the chainstay. Id definitely figure out how to add more clearance than 1mm!
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Old 07-05-22, 08:21 AM
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130 mm spacing on the rear dropouts? 68mm wide BB that BSA threaded. I'd probably just get a 3x Tiagra, Sora or even a Claris crankset with their recommended BB to put on it and be done. Make sure you get the two piece crankset versions. Then you'll be done with the lunacy of square taper fit and won't have that chainline issue.
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