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Thread: Teach me about T47 -- internal, external

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    Default Teach me about T47 -- internal, external

    Running Argonaut cups with Campy on my Alliance, but considering a different crankset to get 165 arms that I can use with my preferred oval rings. It appears that a 24mm axle (to leave more room for internal hydro hose)is available for Rotor cranks and they indicate their axle fits a T47 that is Shimano compatible. As I look at the options out there I see both internal and external bearing versions. I get that one sits inside the BB and the other outside the BB, but I don't know how you determine which is the correct/best for a given crank/BB. Didn't find much via Google that helped with that either. Coming to the collective wisdom here for enlightenment.

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    Default Re: Teach me about T47 -- internal, external

    I used to understand bottom brackets but that was a long time ago. Now I email manufacturers. I tell them what I have and what I want to do.

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    Default Re: Teach me about T47 -- internal, external

    AVT has a pretty detailed description of T47. Typically if you have a 68mm wide BB shell, you use external unless you have a BB30 crank. Also some maker’s BB’s use pressed-in inserts that adapt their 30mm bearing to 24mm. That seems counter the goals of T47 in the first place.

    https://avt.bike/blogs/avt-bike-blog...-to-the-basics

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    Default Re: Teach me about T47 -- internal, external

    Pretty sure Rotor with a 68mm BB shell needs an external T47>24mm BB.

    I say this because Rotor on a 68mm PF30 BB needs spacers either side.

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    Default Re: Teach me about T47 -- internal, external

    in general people make bottom brackets more complicated than they need to be. T47 is less of a "standard" in the traditional sense and more of a package that gives you the flexibility to put the pieces (two bearings and a preload system) in the right place to support a crank. it's a threaded interface like the old BSA bottom brackets, and just like BSA it comes in multiple shell widths. whether you need an internal or external T47 BB will depend on both the width of your shell and the crank you're using.

    24mm cranks are mostly based off of Shimano's Hollowtech II design. SRAM GXP, Rotor 24, and FSA are the common derivatives. They all put the bearings at 82mm apart (center to center) and about 90mm face to face. If you have a 68mm or 73mm(mtb) width t47 shell and want to run 24mm cranks you'll need external cups. if your shell is wider you'll need internal cups.

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    Default Re: Teach me about T47 -- internal, external

    Thanks, everyone. Helpful; I'm equipped to explore options now.

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    Default Re: Teach me about T47 -- internal, external

    One thing to add, T47 doesn't define a specific tool. Depending on which application and manufacture of bottom bracket you could use one of a few different tools. The "torqtite" pattern originally from Enduro is the most common but King does their own. Some of the 24mm bottom brackets will use the old Shimano hollowtech 2 tool (what we call BB Common) but not all of them go that route.

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