I was about to order a new square tapered BB for some folding bike.

Normally I would opt for Shimano UN55, since they tend to last a long time. However I noticed that some new variants were offered: UN300 and UN100.

Since I could not find any detailed info on Shimano's page (nor got a response from the webshop), I ask here.

  • Does anyone have press-releases regarding these UN300 and UN100 parts?
  • Are UN300 as durable as UN55?
  • Do the Chain-case and E-bike variants indicate a lateral offset? (how much?)


After a bit of digging I bring my preliminary conclusion (partly based upon comments), although nothing is confirmed:

  • UN55 and UN26 are no longer available so probably UN100 and UN300 will be the replacements
  • UN300 have a "stabilizer", which is probably just the small plastic spacer on the axle, that will line up flush with the crankset (on the images it looks like UN100 have this too)
  • UN100 is so far only available with 122.5 mm axle, so they probably only target the most-used axle length (and for low-end bikes you might not care much about the chainline)
  • The 3x variants (Normal/K/E) seem to differ in the how the right (drive-side) of the shell looks at the outermost part (see the image). Normal version have more axle exposed, and the E-bike variant have a few mm without threading (to allow for a sensor?). Furthermore the chain-case seems to provide a spacer (see the exploded view)
  • UN300 have aluminium shells on both sides, UN100 have resin shells
  • In terms of price: UN100 6,2 €, UN26 ~8 €, UN300 10,5 €, UN55 15 € (bike-discount.de 2020-06-09)

My guess it that UN300 is a minor upgrade from UN26, by having alu shells and some convenient improvement with spacers (to me it seems unrealistic that new product in this category is 30% cheaper and more durable). I kind of doubt Shimano will introduce a new "high end" square BB on par with UN55.

I think I will buy one of each, and perhaps bring an initial impressions update. Pictures from Shimano's product page lined up (all are 123mm axle)

Update after receiving UN300s and UN100

In general it looks like a production evolution of the previous designs like UN55 and UN26.

TL;DR: UN300 looks to be a well-made unit with good seals, but maybe not as tough as UN55. UN100 looks identical, except for plastic shells.

Let's get the weight out of the way:

  • UN300 270g for a 113mm axle
  • UN100 285g for a 123mm axle... so kind of identical.

The shells look like they are fitted/glued on. On the old UN55 the main unit with drive-side looked as if it was machined from a single piece of steel. The new UN300/100 is 1x steel axle, 2x shells (alu/plastic), 2x cartridge bearings and one middle piece (steel), although the drive-side shell is not immediately removeable.

The bearing on both models are 6903RS (ID17mm, OD30mm). I bought 2x UN300 and 1x UN100, on one UN300 I felt some subtle irregularities/bumps when turning by hand, but I guess that goes away after a bit of use.

The UN300 has the same white powder coating like UN55, and I guess this is similar to blue loctite you find on many bolts. The UN100 does not have this, but for plastic you do not have to care about corrosion anyway.

The finish of UN300 looks great, and has laser etched name + tighten direction.

The shells have seals that attach to the axle and it looks like an effective way of keeping water and dirt out.

  • 3
    Looks like the UN101 and UN300 are Shimano's only current square taper offerings, so presumably replace the UN55. The 300 has far more axle length options. See productinfo.shimano.com/#/spec/MTB/Bottom%20Bracket Commented Jun 8, 2020 at 18:05
  • 4
    This looks like a good write up, but have you put the answer in the question? It's better to add the information into an answer instead, and it's ok to answer your own question!
    – Swifty
    Commented Aug 8, 2020 at 20:14
  • 1
    I just replaced a 68mm/118mm UN55 with a 68mm/122.5mm UN300. The UN300 unit was (surprisingly) over 30 grams lighter than the UN55 unit, despite the slightly longer axle length. The left (non-drive side) aluminium shim screws completely flush with the BB shell (unlike on the UN55, which has a lip). On the UN300 there is a plastic/rubber ring on the drive side of the axle which, IIUC, indicates the position to torque the crank to using the 8mm hex wrench, in order to achieve the specified chainline. I struggled to get my crank arm closer than about 1mm away from this ring.
    – 52d6c6af
    Commented Dec 18, 2020 at 16:35
  • 1
    The axle coating on the UN300 looks of a higher quality than the UN55, but time will tell for sure. There appear to be improved seals on the UN300 unit.
    – 52d6c6af
    Commented Dec 18, 2020 at 16:42
  • 1
    Also: the amount of threaded area on both sides of the unit is much greater on the UN300 unit than it was on UN55.
    – 52d6c6af
    Commented Dec 18, 2020 at 17:03

2 Answers 2


On the german Shimano site the UN300 is a bottom bracket designed for (e-)MTB. It is advertised as available in 3 versions: standard, E-type and closed gearbox (I think it is the spindle&co. without the housing). Different lenghts are offered. It is advertised as part of the "Shimano Acera" family of products.

The UN100 is advertised as part of the "Shimano Altus" and "Shimano Tourney" family of products. It explicitly mentions being compliant with two specifications only (D-NL and LL123)

  • Good point regarding UN100 only being available in one spindle length. I do not get what you wrote about "spindel&co without housing". Honestly i can not use the groups (Altus vs. ...) to determine if UN55 quality is better/worse, since UN55 is not really part of group. Commented Jun 9, 2020 at 17:59
  • @SteffenVilladsen I agree on the group affiliations ... it is just the way it appears on the site and I was mentioning it for completness. One of the 3 versions of the UN300 is the "closed gearbox" version, if I translate correctly from German. I do not know what is exactly the meaning, so I suppose it is only the mechanical parts, without the housing. I don't know.
    – EarlGrey
    Commented Jun 9, 2020 at 18:58
  • 1
    Anyone knows what is D-NL and LL123 in the name of UN300?
    – user66638
    Commented Jan 28, 2021 at 15:42
  • 1
    @user66638 D-NL should be axle-length 122,5 mm, LL123 instead should be 123 mm, but rather than the length, the difference may be related to chainline.
    – EarlGrey
    Commented Jan 28, 2021 at 15:59
  • 2
    LL123 is basically a BB used for cranks with a chain line set to 50mm. For example, FC-M311 or FC-M361. D-NL is used for cranks with a chain line set to 47.5mm. For example, FC-TY501 in the Tourney category. Commented May 16, 2021 at 15:56

BB-UN55 is the first option for its longevity, second place goes to Litepro Titanium BB for evolution and there is the rest of the square tapers...

  • Welcome to the site - OP was specifically asking about the Shimano UNxxx cartridges and the differences between them. This may be deleted as ""not an answer" cos its off on a tangent. You can browse the tour to learn more about how the Stackexchange Q&A format works.
    – Criggie
    Commented Jan 17, 2023 at 11:04
  • You can also edit your post to help add relevancy.
    – Criggie
    Commented Jan 17, 2023 at 11:05

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