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In threaded 2-part bottom brackets there is a plastic top hat/bearing insert/dust cover that goes between the BB's bearing and the spindle, so that they fit tightly and the spindle does not wobble.

The problem is my crankset somehow does not fit the left top hat: it was pushed out from the BB's left half and split by the spindle.

How is it supposed to be installed and not break if it's too tight? enter image description here

I have a Rotor BB1 BSA "Designed to work with ROTOR and Shimano 24mm cranksets" and a Shimano Tiagra crankset.

  • Can you show a photo? I do not understand what you did. Do you men the pastic thingie used to set the bearing preload before fixing the cranks? It should be cheap. – Vladimir F May 7 at 14:23
  • Added a picture. – homocomputeris May 7 at 14:37
  • OK, so not the fixing bolt. I do not know how you managed to crush it (inserting the spindle too fast?) but I do not think it is sold separately. – Vladimir F May 7 at 14:46
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    I think these are sometimes called a top hat – Swifty May 7 at 16:18
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    Are you using a shimano BB or a SRAM/fox? The Gpx ones are the same size as Hollowtech on the drive side (24mm) but are 2 mm smaller on the non drive side, so a Hollowtech crank will fit through the drive side but not through the non drive side. – Andrew May 7 at 17:36
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update: It is also possible that you did not need the spacer/cap in the first space. You state you have a Rotor BB. Didn't you try to use a cap that allows using thinner cranks? For example allows an GPX crank with a Hollowtech II BB. In that case the cap would just be something in the way when using a Hollowtech II crank and so no wonder it would be forcibly and destructively pushed out. The link in the text below show another possibility. A cap that allows Hollowtech II cranks in a wider BB.

Maybe you just must be more careful when installing the crank. I cannot think of any special trick that would have to be done to avoid damaging the caps. If there is any significant resistance, stop and try to fit the crank better. At the and one may need a few hits by a mallet, but that us usually when it already sticks out, just not fully.


I originally thought it was part of the sleeve present in Shimano bottom brackets because the question said "Hollowtech II".

These parts of a Hollowtech II bottom bracket are integral parts of the BB. They connect the opposite bearing cups and separate the bearing from the spindle. You fix them when you screw the left side bearing cup. Then when inserting the crank there should not be any potential to damage it if it was installed correctly. The spindle should go inside reasonably smoothly.

One option for an incorrect install might be using the incorrect spacer in between. E.g. using a 73 mm spacer for a 68 mm bb shell and then screwing the left cup (the force it generates is big). It would then bulge and partially block the way for the spindle. The final damage would be done when forcefully inserting the crank.

Make sure using the correct spacers for your bottom bracket shell on your frame as shown in Difference between Road/Mountain Hollowtech II Bottom Brackets?

There are some spare pieces sold, but not widely available. For example https://www.chainreactioncycles.com/shimano-hollowtech-ii-central-sleeve/rp-prod7428 (this piece is the big one between the cups) and https://www.kineticbikebearings.com/kbbb9037.html (thanks to Swifty for the name). There is usually no need for them so they are not so common in the e-shops. The links are only illustrational. I do not know about your local bike shop, but it is worth asking. You may need a whole new BB.

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  • I also suspect a similar thing could happen with the correctly-sized spacer if the spacer isn't properly aligned so that it engages both cups correctly. – Andrew Henle May 7 at 15:51
  • "Shimano Hollowtech II" the Shimano Tiagra one I've replaced definetely have them. Thanks for update. In both old and new BB, the spindle wobbles inside the bearing. – homocomputeris May 7 at 18:04
  • I may recall.it wrongly. – Vladimir F May 7 at 18:23
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My guess is that the crank spindle was not centered in the plastic spacer when you installed the crank, the edge of the spindle engaged the edge of the spacer as you pushed the crank through (perhaps you used a mallet or rubber hammer???).

You should contact Rotor to find out if they can supply a replacement spacer. They have a contact form on the their web page: https://rotorbike.com/catalog/default/contacts.

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    That is my guess too. – Vladimir F May 7 at 18:43
  • So, it should not be too tight? When I put the crank in, the top hat was pushed out. I put top hat back, held it with my hand, and when the ribbed part of the crank passed through, it started pushing the hat out once again, and then it cracked along the brim. – homocomputeris May 7 at 18:47
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    @homocomputeris, no it should let the spindle pass through, but be quite snug so the spacer grips it. The cylinder part that fits between the spindle and bearing is intact, can you try slipping that on the spindle? Does it fit, or is it obviously too small? – Argenti Apparatus May 7 at 18:59
  • @homocomputeris is there a spacer on the other (drive) side? Does your spindle fit tight in the inner (metal) races of the BB bearings without the spacer? There is a possibility that the spacer was to fit SRAM GXP cranks which are 22mm on the non-drive side. – Argenti Apparatus May 7 at 19:04
  • Yes, there is one on the drive side. – homocomputeris May 8 at 9:40

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