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I have the bottom bracket shown below, that needs replacement. The bike is from 2014.

enter image description here

I live in Argentina, and here I see plenty of 113mm x 68mm in the market. But I cannot get a 113mm x 73mm as the original.

I have been told at two LBS that 68mm would work fine, I have to screw first the drive side of the BB, and then the other side until it hits the ball bearing.

Is that information reliable? I am not sure if it depends on the bike, if it may be less durable, etc. But as of now, my only choices seem to be using a 68mm BB, or looking for someone that can bring the BB from abroad (shipping is very expensive).

In addition, something that puzzled me is the fact that at a LBS, I got one off-branded BB spec'ed 68mm and I measured the span shown in the figure (which is the size in the spec) with a caliper, and it was 73mm! Is there any reasonable explanation, other than it is wrongly labeled? Note: the image below is taken from the web (it is a Shimano, and labeled 73), it is not the BB I measured.

enter image description here

Related:

  1. https://www.sheldonbrown.com/gloss_n-o.html#oversized
  2. https://bicycles.stackexchange.com/a/38207/24190 advises against using 68mm.
  3. Part of this answer belongs in here, and it also advises against using 68mm.
  4. https://www.sheldonbrown.com/bbsize.html, table below "Bottom Bracket Threading"
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2 Answers 2

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It is probably correctly labeled. The inscription on the BB refers to the size of the BB shell it is meant for. Not the measurement you just made. Here you can see an old BB UN300 68mm, which also measures 73 mm in your way of measuring. It is however still made for a 68mm shell, as stated on the BB.

Since the lip of the driveside sits on the outside of the BB shell, the measurement in your pic would almost certainly be greater than 73mm. enter image description here

I doubt it is a good idea to install a 68mm in a 73mm shell (have you measured your shell width to be 73mm or only looked at the BB you removed??). It might get in there, but it will probably not get proper support or preload and will probably fail quickly. (This is just my opinion, I have no experience with installing a 68 BB in a 73 shell. Perhaps you can insert the non drive-side the extra 5mm IF there is enough thread depth, but it would not sit as intended and it sounds fishy/risky.)

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  • Thanks for the practical demonstration! Note that the 2nd image I posted is taken from the web, it is not the BB I measured. I didn't measure the shell in my bike, I will do it as soon as I have access to it again in a couple of weeks. I am only learning about all these details.
    – sancho.s ReinstateMonicaCellio
    Commented Jan 27 at 23:46
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    @sancho.sReinstateMonicaCellio You are welcome! I understood the pic was from the web, I just wanted to point out that the pic is misleading/wrong and caused your confusion. A 73mm BB would probably measure something like 78mm when measured the way in the picture.
    – WornChain
    Commented Jan 27 at 23:58
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Your LBS is technically correct in that a 68mm BB can be used in a 73mm shell (given the threading of cups and the shell match, ie: BSA threaded BB in a BSA threaded shell). There are many reasons to NOT use a 68mm BB in a 73mm shell. In many cartridge bearing BB's, the right (driveside) cup is fixed to the cartridge (bearing assembly) and is also flanged. That acts a stop. When a designed 68mm BB is placed in the 73mm shell, it will be shifted, relative to the frame centerline (which is a point from which the chainline is determined) 2.5mm to the right, driveside. This will move your front crank's chainline 2.5mm outbound. The chainline increases by 2.5mm. Front derailleurs work best with a chainline between 47.5-50mm. Question becomes can the derailleur move out enough to get the chain on to the outer, large chainring with the wider chainline? Smoothly and consistently no less.

While that is being considered, at the non-drive side, the left crankarm is gonna be 2.5mm closer to the frame since all things to do with the BB have shifted 2.5mm toward the drive side using the shorter, 68mm BB in a 73mm shell. Question that will need clarification before riding is, does the left crankarm have adequate clearance of the frame now (specifically the non-drive chainstay)?

Also at the left in this situation, is checking to make sure the left cup of the 68mm BB can thread far enough into the 73mm shell to adequately support the cartridge bearing--spindle assembly AND adequately preload the bearings. While many left/NDS cups come flangeless and can be threaded past the outside edge of the shell, the splined tool fitted into the splined outer aspect of the left cup needs adequate engagement to reach proper torque. In addition the cup may reach the end of the shell threads before the system has adequate preload.

In short, there's so many gotchas, that it becomes not worth it. Regarding the discrepancy of the spindle length of the proper, 73mm BB, it's best to go a little longer. Again, you may change the chainline with this approach so you don't want to go a lot over. Shimano does make a 73/113 BB-UN300.

One hack you can do (and this is not the best thing) is using the cups from your current 73mm BB. You'll have to remove the non drive side cup and the old spindle and bearings. Then, take your new 68/113 BB and remove its right cup off the spindle/cartridge bearing assembly which you can then insert into your shell (mindful of the correct orientation of it). You may have to tap it into the right cup of the old BB which is still installed in the shell. Secure with the old left cup. This hack has its pitfalls as well, including possible damage to the new cartridge. Also if the new BB's right cup is fixed to the cartridge with more than a press fit, you likely won't be able to remove it without damaging the cartridge. So....

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  • What do you mean by preloading? Does the left hand cup take the weight off the bearing and let it roll freely?
    – Ender
    Commented Apr 25 at 19:58
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    Preload is actually the application of axial force to the bearings that removes excess play in the system. Preload brings the races into proper alignment and reduces free space between the rolling elements. If this free space is not taken up by preload the races can become misaligned, the balls can slide or skid instead of roll, resonant vibration can lead to abnormal wear and noise. Preload improves the stiffness in the system which creates rotational accuracy because the races are aligned and the balls have just enough space to roll but not otherwise "move."
    – Jeff
    Commented Apr 26 at 1:55
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    The left hand cup of a cartridge bottom bracket provides mechanical preload to the bearing system. It aligns the races and closes the clearances which creates stiffness and rolling precision. With external BB's for 2 piece cranks (Shimano Hollowtech2 and others, the newer type of crankset systems) preload is applied by rotating a plastic nut into the spindle from outside the left crankarm. This draws the left crankarm further onto the spindle which places axial force on the outer aspects of both external bearings. Other systems use a wave washer an initial preload.
    – Jeff
    Commented Apr 26 at 2:06
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    Lots of issues here. The deviation caused by cross threading can cause misalignment and uneven preload. It would certainly be best to remove the cross threaded BB and replace with a proper 73mm one. If the cross threading hasn't completely destroyed the threads, one can use a pick and patience to clean up the threads--essentially removing or moving material out of the thread valleys and restoring the bent peaks as best as possible. Careful insertion of an undamaged cup will "chase" the threads and restore them even more cleanly. Taps for BB shells are not common except in bike shops (maybe).
    – Jeff
    Commented Apr 26 at 21:01
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    A tap to chase the threads would be best of course. That's if there is enough material left to rethread to the same size. If the shell threads are too damaged, some folks tap the shell to the slightly larger (than BSA) Italian thread spec and then use the appropriate Italian threaded BB in the shell. Most cartridge BB's are offered in both the BSA threaded (most common) and Italian threaded versions. Same thing, just different thread size and pitch.
    – Jeff
    Commented Apr 26 at 21:06

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