I'm fixing a bike which has BSA BB shell (right hand thread on NDS, left hand thread on DS). But the shell width is not 68mm. My measurement shows it's about 69.8mm ~ 70mm.

I assume original BB is BB-1050 and manual page says shell width is 70mm source, so I guess my frame was built with 70mm BB shell intended.

I was going to replace it with Shimano Tiagra BB (BB-RS500), but it seems 70mm option comes with Italian adapter. So I think it might not be applicable to my frame.

Is there any option I can take? I'm going to install Shimano Claris crank set (FC-R2000). Reading this SE answer, it sounds like I can just use 68mm BB-RS500, but I'm not sure it really works.

  • 4
    Is your bike an older Raleigh? Stop before threading anything in if so! Waiting for your answer.
    – Noise
    Aug 25, 2022 at 13:38
  • @Noise I don't know anything about the frame, it is old enough though (literally every label has been removed when I received it). It looks like classic bike and it has quill stem.
    – xvnm
    Aug 25, 2022 at 14:22
  • 3
    Read through this page and see if it applies to you. I had one drop bar bike through with this problem and it took a while to work out what was going on. sheldonbrown.com/raleigh26.html
    – Noise
    Aug 25, 2022 at 14:56
  • @Noise Thank you. reading from the website, I think my bike is not Raleigh. Dropout is not the shape.
    – xvnm
    Aug 25, 2022 at 15:33
  • 1
    The dropout is not a sure tell, the road bikes didn’t have it. Aanyway, you have the information you need, hope it goes well.
    – Noise
    Aug 25, 2022 at 17:09

2 Answers 2


You say it's BSA and that it's DS LH, NDS RH thread, but you don't corroborate what exactly the thread size is. It will be easier to get to a good result here if you know what the thread is. It's easier with a thread pitch gauge and caliper, but perfectly doable to the extent needed here (ie telling the difference between 24 and 26tpi) with a ruler.

As has been pointed out in the comments, the first question is whether it's a Raleigh thread (26tpi instead of 24). You should really measure to get that sorted out.

If it had FC-1050 in it before you can't assume the BB was BB-1050 or any other particular BB. BB-1050 has its model name and thread stamped very clearly on it so unless it's long gone or something, it sounds like they may not be what it was. Meanwhile, as in the Sheldon link, re-using the cups and transplanting in a JIS spindle is a very good way of addressing BB/crank type repairs on Raleigh shell bikes, so it's plausible that was done at some point.

If it's a 70mm, 34.8mm x 26tpi Raleigh shell and the goal is to install 24mm Shimano road cranks, the main question is whether you're up for first confirming the cranks you want will clear the rest of the frame once you make the necessary modifications to run them, then grinding/filing and refacing the shell down to 68, then acquiring the Phil Wood 26tpi cups to do it.

If it's 70mm, 34.8 x 26tpi Raleigh and you don't want to jump through those hoops, then keeping it square taper is the clear choice. If you want to make it cartridge you can use the Phil cups and BB for that. If it needs to be a lower end repair, serviceable Raleigh cups are not scarce in the world, as Sheldon pointed out, but they'll need to be acquired used/vintage.

If somehow it is 34.8mm x 24tpi but has a 70mm shell width, that makes it a random oddball. The main consideration becomes that external road cranks with ISO/BSA BBs generally speaking don't just have 2 extra millimeters of space to give like that. The spindle won't be fully engaged into the left crank on a Shimano road crank, for example, which may seem to be fine in the stand but isn't necessarily safe. Depending on the crank, you can also have issues with the bearings not contacting the proper machined areas of the spindle, causing play or creaking. Italian external BBs use thinner cups to get to the same face-to-face dimension, but that doesn't help you. The straightforward thing to do again would be check the clearances, file the shell down to 68, and face it. If that was an unattainable solution, the only thing I can think of that allows you to put new cranks on and have everything be arguable as technically "right" is just use a Phil square taper BB, which are uniquely oblivious to the exact shell width because they can be inset on both sides. Alternatively, again if that's all too fancy, you could run a normal 73mm cartridge, put a 1.5mm spacer on the DS, and see how it does being down 1.5mm of thread engagement per side on the cups. (You would find a square taper road crank and use whatever spindle length would normally be called for with your chainline needs, only you'd be getting the 73mm shell version).

  • Thanks. I measured TPI and it was 24 (12 teeth on 1/2 inch), so I've got random odd ball ;). I will try 68mm BSA BB, better linked SE answer is right.
    – xvnm
    Aug 26, 2022 at 2:59
  • 1
    @xvnm it WILL appear to work when you do that, but understand that the spindle won't be engaged into the left crank as much as intended by the manufacturer, which could lead to issues with security. Aug 26, 2022 at 3:26
  • "The spindle won't be fully engaged into the left crank on a Shimano road crank, for example, which may seem to be fine in the stand but isn't necessarily safe." If the safety pin on the NDS crank arm fits into the small spindle hole, I'd personally would think it's safe enough to ride. But then again, I am not a wattage bazooka. :) Aug 29, 2022 at 15:54
  • @Superman.Lopez it doesn't matter if you aren't putting out pro watts. With your hands in the stand, I suspect you can put 5-10 watts into the cranks. You could easily average 100W on a casual ride, and hard efforts by even an amateur should be 200 or more (or much more). So I, too, would assume those 2mm matter.
    – Weiwen Ng
    Aug 29, 2022 at 16:42
  • Yes, but all products are manufactured around tolerances. And if you constantly apply a lot of force, you will go beyond those tolerances sooner than when you apply lower forces. Same is the reason why I would personally take the risk if the safety pin fits. These cranks are build around tolerances in shell width, and the safety pin is a design to ensure the shell width meets Shimano's tolerances. So if it fits, I'd be inclined to trust it. Aug 29, 2022 at 16:55

Following the conversation in Nathan Knutson's answer, I think you concluded it's a 70mm shell width by 34.8mm and 24tpi.

Personally I'd try to get an inexpensive 68mm Hollowtech BB (eg. second hand) and see if the Claris crankset could fit. My own criteria would be to see if the NDS crank arm safety pin could fit in the hole of the spindle. To me that would be an indication that the spindle sits deep enough in the NDS crank arm.

If it doesn't fit and you are looking to source something easy and not too expensive, you could consider getting a Shimano MTB BB and a MTB crankset. These are easy to source and typically come with 3 spacers of 2.5mm to make it work on a 68mm shell. if you'd use 2 instead of 3 spacers, you'd get quite close to the required width.

  • Both solutions are of course not within Shimano specification, and are therefor a hack/botch... Also the MTB BB would be threaded deeper into the shell than on a 68mm shell, but in my experience the shell has a bit more thread than the BB. YMMV! Aug 29, 2022 at 16:31
  • 1
    Nice advice. Somehow 68mm Hollowtech BB and safety pin fitted, so I too decided to trust the interface. (also, I'm not going world tour with this bike)
    – xvnm
    Aug 31, 2022 at 12:52
  • Good to hear it still fitted. And I assume the cranks still spun freely? All the best and hope it doesn't have any unexpected results! Aug 31, 2022 at 17:18

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