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I wish to convert my 1 piece crank to a 3 piece crank. I have a conversion kit that presses into the bb shell and accepts a cartridge style bb. I went to order one and realized I have no idea what spindle/axle length to get and wouldn't have a clue how to get this measurement from the one piece crank. I have an idea how to get it using the original chain line measurement which falls in at around 45mm but would like to know if there's a more accurate or formal way of determining the bb I should get for this job. I have a couple of options for crank arms in 170 & 175mm length and either square taper or Octa-link and whatever truvative/Sram calls their similar interface.

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  • So i asked the guys at the local shop i use and besides trying to get me to bring it in so they could take a look at it and try to charge me an arm, leg, and my first born to do it for me, they suggested a method using the distance between the pedal mounting surfaces of the crank arms which means i basivallu pick any 68mm cartridge and look up crank arm specs to find as match or by trial and error? That doesn't seem very senseful so i started looking around for more information and found a different setup entirely. Instead of converting to a cartridge it's a complete set including a spindle. Aug 8, 2023 at 11:00
  • But I still don't know nor can i find how to determine the length of the spindle/axle. What am i missing here? This can't be something that nobody has info on. I couldn't even find anything on Sheldon Brown's site. All the bike guys on YouTube, nothing. What's the deal with this? Aug 8, 2023 at 11:14

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If going with the adapter which interfaces an American BB shell with a sealed cartridge BB, 68x113 works with most standard mtb crank arms.

You'll also need to stick with mtb chain ring setup.

The other conversion kits which are specifically made for BMX conversions may or may not be a complete kit with crank arms ans a sprocket however spindle length isn't a factor when choosing one, they are all a standard size. I don't know what that is specifically because I went with what I already have which was the adapter and mtb parts mentioned above. However since it's not a factor when shopping for one I guess that doesn't matter.

What you have to know when shopping for one of these kits is whether it's for an American, mid, or European (French, Italian, etc) BB shell, spindle diameter (19, 22, or 24mm. 19 is most standard duty, 22 for heavy duty, 24 for race/competition. I also read that there are 2 different specs for 24mm, one being actually 23.xx and one being true 24mm for race only) crank arm length, and sprocket tooth count (last 2 for a kit that's complete with arms and sprocket.).

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