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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 07-17-14, 02:39 AM   #1
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The Bottom Bracket Conundrum

Hi all!

(Background that you can mostly ignore if you'd like)

So I'm trying to replace the bottom bracket on a new bike that I'm building, but I need a bit of help. The main idea is this: I have a GMC Denali (which runs fine-ish) that I disassembled as I prepared to clean and repaint it, but recently got my hands on an older, lighter Fuji frame. So I decided I'd just transplant as many of the parts as I could from the Denali to this new frameset.

(The actual conundrum)

One problem I'm running into is that the Fuji doesn't have a bottom bracket, and I'm not quite sure on the specific of what I should get to replace it to use the crank arms from the Denali that I already have, and I don't know the model of this frame because I can't trace the serial to anything online (and I think part of the serial is worn away). What I do know: A usable replacement for the Denali measures 68mm BSA and 122.5mm, aaccording to the fellow that posted this video here.

So I'm pretty lost as to what I should do. Should I buy a new bracket? If so, how do I identify what taper I need, what length I need, what other parts I need, etc? Can I transplant the original bottom bracket? That will require a 20 dollar tool that I'd have to purchase off Amazon, because of ring attachments that hold the BB in, and if so, can I just buy a new bracket and install it for cheaper than buying that tool?

If anyone can guide me through the general process of what to look for when replacing a BB (how is it done? Does it depend solely on the crank set used, or the frame as well? Is the BB shell size usually standard? etc.)

I'm very much a newbie here, and googling has basically yielded "take it to your bike shop so they can fit the right BB." Any help would be appreciated.
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Old 07-17-14, 06:54 AM   #2
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The odds greatly favor the Fuji also having a 1.37"x24 ISO bottom bracket. The only real question is the the width. Most are 68mm wide but 73mm isn't all that rare, so start by comparing the width of the BB shell (frame) of both bikes. If they're the same, you can transfer the BB and cranks. If not, you can buy an appropriate BB for the Fuji, with the same spindle length and type as the Dinali. As for things like the type of taper, don't sweat it, you're not likely to run into the wrong taper unless you shop high end exotic bottom brackets costing more than your bike. You're concern is whether it's a typical square taper, vs some kind of spline drive like these two.
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Old 07-17-14, 01:16 PM   #3
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Whatever you do you will need a tool to remove the crank from the old bottom bracket and to install the bottom bracket into the Fugi frame. If you choose to use the very low quality bottom bracket from the Denali you could possibly install it using tools that you might have at hand, but be aware that it might fail fairly soon. If you choose to install a better quality cartridge type bottom bracket you will need the appropriate tool
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Old 07-17-14, 01:31 PM   #4
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My recommendation is purchase a UN55 bottom bracket (68x120ish but heed FBinNY and measure first) and the bottom bracket tool and discard the Denali bottom bracket.

If you keep the Denali one you'll need to be greasing it periodically, and would probably want a spanner for the cups for that since it's a huge pain using a big adjustable spanner (or whatever you have). So you might as well get a Shimano BB tool instead and a sealed cartridge BB.
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Old 07-17-14, 06:13 PM   #5
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Thank you all so much. I've gone ahead and decided that I'm going to keep the frameset and use it for a second bike, but this has been helpful anyway, since bottom brackets continue to confuse the crap out of me. I'm definitely leaning towards getting a sealed bottom bracket, since I actually already have a bottom bracket tool for Shimano for whatever reason.

I'm curious, how does one know exactly what bottom bracket would fit in a bike? Is there a standard size that usually fits in and then a bunch of alternatives for higher end or more specialized bikes?

Also, is there any harm to using a 73mm when the necessary spec is 68? Wouldn't that just leave a bit of excess on the bracket? Not that I plan to--again, mostly for personal curiosity
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Old 07-18-14, 08:19 AM   #6
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The BB spindle length has to match the crankset, but has to be the proper threading for the frame. Most modern frames use ISO (or British) threading, and most road frames have a 68mm BB shell. Using a 73mm BB in a 68mm shell would throw your chainline off.

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