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Can I use the left cup from a 118 mm BB-UN55 on my 107 mm BB-UN55?

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Can I use the left cup from a 118 mm BB-UN55 on my 107 mm BB-UN55?

Old 09-18-21, 06:18 AM
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mikael_on_wheel
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Can I use the left cup from a 118 mm BB-UN55 on my 107 mm BB-UN55?

I have a 107 mm BB-UN55 bottom bracket without a left cup. Can I use the left cup from a 118 mm BB-UN55? Are they the same? Bikester[1] (and many other places) sells what looks to be a axle length specific 107 mm left cup/shell for the BB-UN55. Are the cups different depending on axle length for the BB-UN55?

Valuable frame (to me), so I'd rather not do something damaging.

[1] Can't post a link as I'm a new member.
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Old 09-18-21, 07:34 AM
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If the thread is the correct one shouldn't be a problem
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Old 09-18-21, 09:24 AM
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If you have it and everything else there in front of you, try it and see. Do you know how the other one fit? Make sure this fits the same.
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Old 09-18-21, 09:33 AM
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Shimano's Exploded View for the BB-UN55 says those two left adapters have different part numbers. I'd guess that means "no".

To vtiew he document directly from Shimano, Google "exploded view BB-UN55" and select the entry from "si.shimano.com" in the results. You'll get a PDF format version of the document.

I'd post a link, but Google now makes it very difficult to do that for PDF documents with my browser (Firefox) if you have it set up to use Acrobat Reader as the default reader. But the above search is pretty easy to do, and when I searched the Shimano link is one of the first handful.

Last edited by Hondo6; 09-26-21 at 06:37 AM. Reason: Correct typo and omission.
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Old 09-18-21, 09:56 AM
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As far as I know, the cartridge bodies all have the same dimensions at the end (OD, edge radius, etc.) Problem Solvers sells a universal replacement cup in each shell width (68mm or 73mm) and it's not specific to the bottom bracket length.

The differing part numbers may indicate whether the left-side cup has a lip on it or not, I much prefer the ones without.

EDIT: I was right! The cup shipped with the 107mm unit doesn't have a lip (it's threaded all the way to the end): https://www.amazon.co.jp/-/en/SHIMAN.../dp/B008E4KA90



The other cup has a lip on it: https://www.amazon.co.jp/-/en/SHIMAN.../dp/B008E4KAJA

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Last edited by ThermionicScott; 09-18-21 at 10:00 AM.
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Old 09-18-21, 10:50 AM
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Originally Posted by ThermionicScott
As far as I know, the cartridge bodies all have the same dimensions at the end (OD, edge radius, etc.) Problem Solvers sells a universal replacement cup in each shell width (68mm or 73mm) and it's not specific to the bottom bracket length.

The differing part numbers may indicate whether the left-side cup has a lip on it or not, I much prefer the ones without.

EDIT: I was right! The cup shipped with the 107mm unit doesn't have a lip (it's threaded all the way to the end): https://www.amazon.co.jp/-/en/SHIMAN.../dp/B008E4KA90



The other cup has a lip on it: https://www.amazon.co.jp/-/en/SHIMAN.../dp/B008E4KAJA

Interesting. Presence/absence of a lip would explain the different part numbers.

Makes me wonder why Shimano didn't simply use the same left adapter within each size they made (68mm wide, 73mm wide, and 70mm wide), though. The 107mm left adapter is different from the other spindle lengths for each size. Seems to me it would have been significantly cheaper and easier for Shimano to make 3 different left adapters for the BB-UN55 series vice 6.

Seems to me they'd have a good reason for fabricating 6 different parts, given the extra expense involved, rather than 3. But maybe not.
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Old 09-18-21, 10:59 AM
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It is easy to check before you install it by sliding it over the bb, to make sure it fits, and measuring the width when bottomed out to see how it fits the shell width.

In all honesty, the left (NDS) retainer doesn’t do a whole lot. I imagine there is some minimal support provided to the cartridge; although minimal is being somewhat generous when talking about a plastic retainer.

The best thing it does is help keep crud out of the bb shell and the threads clean.

John
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Old 09-18-21, 11:11 AM
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Or just buy a complete bb-un300 of the appropriate spindle length. It'll look more like the bb-un52 that might have been in that bike originally.
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Old 09-18-21, 11:38 AM
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Originally Posted by Hondo6
Interesting. Presence/absence of a lip would explain the different part numbers.

Makes me wonder why Shimano didn't simply use the same left adapter within each size they made (68mm wide, 73mm wide, and 70mm wide), though. The 107mm left adapter is different from the other spindle lengths for each size. Seems to me it would have been significantly cheaper and easier for Shimano to make 3 different left adapters for the BB-UN55 series vice 6.

Seems to me they'd have a good reason for fabricating 6 different parts, given the extra expense involved, rather than 3. But maybe not.
Yeah, I don't have a good answer for that! Perhaps Shimano felt the longer lengths needed a little more support against the bottom bracket shell, but it has never been a problem when I've used the lipless cups on longer units. Plus, if the BB shell isn't exactly 68mm wide, the lips won't make contact anyway, rendering them superfluous...

Originally Posted by 70sSanO
It is easy to check before you install it by sliding it over the bb, to make sure it fits, and measuring the width when bottomed out to see how it fits the shell width.

In all honesty, the left (NDS) retainer doesn’t do a whole lot. I imagine there is some minimal support provided to the cartridge; although minimal is being somewhat generous when talking about a plastic retainer.

The best thing it does is help keep crud out of the bb shell and the threads clean.

John
Be that as it may, if the left cup isn't installed in a way that supports the unit, then all of the pedalling force from your left foot goes into the right-side cup. I could see that being damaging to the unit, maybe distorting the right-side cup or causing the bearing to bind.

That's why when someone uses a left-side cup with a lip and frets about a small gap when it's torqued down, I try to talk them out of using spacers to take up that gap. As installed, it's providing as much support as it can to the unit. If they add too much in the way of spacers, the left-side cup will bottom out against the bottom bracket shell before it can fully engage the unit, and so it won't support it properly.
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Old 09-18-21, 11:59 AM
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Originally Posted by Iride01
Or just buy a complete bb-un300 of the appropriate spindle length.
Except for the whole supply chain "out of stock" thing going on.
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Old 09-18-21, 01:10 PM
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Originally Posted by dedhed
Except for the whole supply chain "out of stock" thing going on.
Seem to be in stock here in 107mm spindle length and 68mm BB width:

https://www.treefortbikes.com/Shiman...ish-68mm-Shell
https://northwestbicycle.com/product...spindle-cr3482
https://www.modernbike.com/product-2126306132

Last edited by Hondo6; 09-18-21 at 01:21 PM. Reason: Add additional URLs.
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Old 09-18-21, 10:09 PM
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Better yet, if OP really loves the frame, buy a Tange Seiki LN-3922 (made in Japan):

Tange LN-3922 Bottom Bracket | - Ben's Cycle (benscycle.com)
Tange LN-3922 Square Taper Bottom Bracket – The Bikesmiths

I recently bought one (68 mm shell, 107 mm spindle length) from Ben's Cycle for my 1X conversion:

Converting Trek FX2 to 1x - Bike Forums

The Tange Seiki LN-3922 is an excellent bottom bracket.
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Old 09-19-21, 05:44 PM
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Originally Posted by SoSmellyAir
Better yet, if OP really loves the frame, buy a Tange Seiki LN-3922 (made in Japan):

Tange LN-3922 Bottom Bracket | - Ben's Cycle (benscycle.com)
Tange LN-3922 Square Taper Bottom Bracket – The Bikesmiths

I recently bought one (68 mm shell, 107 mm spindle length) from Ben's Cycle for my 1X conversion:

Converting Trek FX2 to 1x - Bike Forums

The Tange Seiki LN-3922 is an excellent bottom bracket.
Tange makes excellent components. I'd have zero qualms about using one of those, either.
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Old 09-19-21, 07:19 PM
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The LN-7922 is nice as well, and lighter. (If you can get your hands on one in the length you need at the moment.)
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Old 09-19-21, 07:44 PM
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Originally Posted by ThermionicScott
The LN-7922 is nice as well, and lighter. (If you can get your hands on one in the length you need at the moment.)
But $20 more for that hollow spindle!

Tange-Seiki LN 7922 JIS cartridge bottom bracket – Retrogression (retro-gression.com)
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Old 09-19-21, 08:02 PM
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Originally Posted by SoSmellyAir
Yeah, they were cheaper when I bought mine a few years ago. Crazy times...
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