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Cartridge Bottom Bracket Swap Help

Old 08-22-20, 10:34 AM
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bamab2
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Cartridge Bottom Bracket Swap Help

Here's the deal. I'm doing a little work on the High Sierra I picked up a while back. The BB spindle is pitted pretty bad. I am sort of hoping there is a cartridge BB that I can swap to. This is my first time to do any replacing of BB parts (other than bearings).

Does anyone know of a cartridge style BB that can be used?

Here is what I know:
68mm shell
The current spindle is stamped 3U, D-6, and SCM.
The spindle measurements don't quite match Sheldon Brown's site.
My measurements (approx) are as follows:
Spindle length is 127.5mm
Left side is 32mm
Center is 52mm
Right side is 40.5mm

Help would be appreciated.
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Old 08-22-20, 11:20 AM
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dddd
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Those spindle numbers suggest that you could mimic the existing setup with a 127.5mm symmetrical cartridge with a 4mm spacer under the driveside cup.

I find that in practice, usually there is too much of a driveside offset to the OEM chainline, so the same 127mm cartridge might work with just a 1mm spacer or perhaps none at all if the chainrings clear the chainstays and if the front derailer still pulls the chain down to the smallest ring under some degree of pedaling torque.

1) Did you note how the chainring clearance to the chainstays looked before pulling it all apart?

2) Is there much leeway to loosen the lo-limit screw and still have the front derailer retract significantly further in?

These two question's answers will inform as to whether a common 127.5mm symmetrical cartridge bb will be the best choice.
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Old 08-22-20, 04:42 PM
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Originally Posted by dddd View Post
Those spindle numbers suggest that you could mimic the existing setup with a 127.5mm symmetrical cartridge with a 4mm spacer under the driveside cup.

I find that in practice, usually there is too much of a driveside offset to the OEM chainline, so the same 127mm cartridge might work with just a 1mm spacer or perhaps none at all if the chainrings clear the chainstays and if the front derailer still pulls the chain down to the smallest ring under some degree of pedaling torque.

1) Did you note how the chainring clearance to the chainstays looked before pulling it all apart?

2) Is there much leeway to loosen the lo-limit screw and still have the front derailer retract significantly further in?

These two question's answers will inform as to whether a common 127.5mm symmetrical cartridge bb will be the best choice.
I didn’t measure before disassembly but I reassembled and measured. There is slightly less than 5.5mm clearance between the small chainring and the chainstay but I adjusted the limit screw out and it doesn’t quite move enough pull the chain off the small chain ring.

If I go with a 127.5mm symmetrical cartridge, wouldn’t a 2.5mm spacer on the drivers side bring that side almost back to original location? There is enough clearance on the NDS. Am I doing the math correctly?

I assume there is not a BB with a longer spindle?
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Old 08-22-20, 05:30 PM
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Considering the 8.5mm difference between your spindle's left end right ends, it woud take exactly 4.25mm of mounting offset (= to cup spacer thickness) in order to make a symmetrical bottom bracket have the same 8.5mm difference.

But with some leeway (perhaps 3mm) to move the new symmetrical bb toward the left, you might need only a 1mm spacer under the driveside cup flange and still have chainring clearance and hopefully also strong FD function.
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Old 08-22-20, 06:31 PM
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Originally Posted by dddd View Post
Considering the 8.5mm difference between your spindle's left end right ends, it woud take exactly 4.25mm of mounting offset (= to cup spacer thickness) in order to make a symmetrical bottom bracket have the same 8.5mm difference.

But with some leeway (perhaps 3mm) to move the new symmetrical bb toward the left, you might need only a 1mm spacer under the driveside cup flange and still have chainring clearance and hopefully also strong FD function.
Thanks for replying dddd. I now realize I had a typo. My NDS measured 35mm. Sheldon’s chart showed the 3U as 124.5 length and 32 on the NDS. I guess I was thinking of that and typed 32.

You were telling me right according to the info I gave. With a difference of 5.5 between the two ends, 2.75 would be the equalizer. So a 2.5 spacer should be pretty good, right?
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