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Old 05-25-11, 09:30 AM   #1
Mojo GoGo
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External BB to square taper - possible?

Does anyone know if a bottom bracket / bearing kit exists for converting an bike that has external bearing cups (Shimano 6600 series) to an older square taper design bottom bracket?

I know this is a "backwards" conversion but I've got a old Specialized Allez frame with a square taper BB that I'm running as a single speed. It was my first bike and the shop I got it at sized me incorrectly (it's a 50 cm and I should be a 52 or 54). I also have a Trek Madone 5.2 frame (unfortunately just hanging on the wall) that is the correct size. Wondering if there is any way I can move my current crank over with a BB conversion kit rather than having to buy a crank and bearings.

Thanks in advance.
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Old 05-25-11, 09:36 AM   #2
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Originally Posted by Mojo GoGo View Post
Does anyone know if a bottom bracket / bearing kit exists for converting an bike that has external bearing cups (Shimano 6600 series) to an older square taper design bottom bracket?

I know this is a "backwards" conversion but I've got a old Specialized Allez frame with a square taper BB that I'm running as a single speed. It was my first bike and the shop I got it at sized me incorrectly (it's a 50 cm and I should be a 52 or 54). I also have a Trek Madone 5.2 frame (unfortunately just hanging on the wall) that is the correct size. Wondering if there is any way I can move my current crank over with a BB conversion kit rather than having to buy a crank and bearings.

Thanks in advance.
You just need the proper bottom bracket for the square taper. The external bearings thread into the bottom bracket shell of the bike with the same thread pitch and direction as a cassette bearing.

You can't use the external bearings, however. Different system, diameters and spindle length.
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Old 05-25-11, 09:42 AM   #3
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Strip the frame and start over, as said above.

Unscrew the external bearings,
they were machined to screw into BB's as they were made, right is LH thread.

you Will need special tool to grip the external bearing cup,
to unscrew them, so if there is no desire to buy that tool, to use once,
get the bike shop to use theirs.
the frame threading is unchanged, so you then install a square taper BB .

package the external bearings with the crankset they came with.

only a BB 30 set is needing a unique frame.

Last edited by fietsbob; 05-25-11 at 09:49 AM.
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Old 05-25-11, 09:55 AM   #4
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You just need the proper bottom bracket for the square taper. The external bearings thread into the bottom bracket shell of the bike with the same thread pitch and direction as a cassette bearing.

You can't use the external bearings, however. Different system, diameters and spindle length.
So if I understand you correctly (I haven't looked at Madone frame in a year), the external bearing cups thread into the inside of the bottom bracket. So the square taper bottom bracket I'm currently running just needs to be removed from my Allez and should screw into the Madone frame no problem.

(just assumed some sort of backward incompatibility (e.g., external cups having external bottom bracket threading) had been designed in by the manufacturer)
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Old 05-25-11, 10:02 AM   #5
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Originally Posted by Mojo GoGo View Post
So if I understand you correctly (I haven't looked at Madone frame in a year), the external bearing cups thread into the inside of the bottom bracket. So the square taper bottom bracket I'm currently running just needs to be removed from my Allez and should screw into the Madone frame no problem.

(just assumed some sort of backward incompatibility (e.g., external cups having external bottom bracket threading) had been designed in by the manufacturer)
Yes, they do. The external cups also thread into the same internal threads. Your assumption that they fit onto some external threads is where you are wrong. Unless it's a BB30 frame as mentioned already your frames are both fully compatible with each style of BB.
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Old 05-25-11, 10:40 AM   #6
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Quote:
Originally Posted by Mojo GoGo View Post
So if I understand you correctly (I haven't looked at Madone frame in a year), the external bearing cups thread into the inside of the bottom bracket. So the square taper bottom bracket I'm currently running just needs to be removed from my Allez and should screw into the Madone frame no problem.

(just assumed some sort of backward incompatibility (e.g., external cups having external bottom bracket threading) had been designed in by the manufacturer)
The only issue may be the spindle length. I'd suspect that's not even going to be much of a problem.
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