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Old 09-07-11, 03:36 AM   #1
trdsupragt
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How to choose bottom bracket?

I'm in the process of updating my front drivetrain, and in the event my FD doesn't agree with my current crankset, I'm considering upgrading to an Ultegra 6500 crankset that my friend will sell to me for dirt cheap.

The problem is that I don't know if my current BB will work, and if it doesn't, which bottom bracket will I need to match the Ultegra crankset? Will the newer bottom brackets even fit my frame? It's a '90 Trek 1000. I'm guessing since it's an American bike, I'll need an english BB? Or do BB's get more complicated than that?

Last edited by trdsupragt; 09-07-11 at 03:56 AM.
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Old 09-07-11, 06:24 AM   #2
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If I remember correctly, 6500 used an Octalink-style bottom bracket, so you'll need something like this:

http://www.universalcycles.com/shopp...7&category=347

You'll need to make sure you get the right size. A 90's Trek is probably a standard 68mm English shell. Note that you'll probably also need the appropriate splined tool for isntalling the bottom bracket, something like a Park Tool BBT-32C.

The width of the spindle will depend on whether it's a 2 (109mm) or 3 (118mm) speed crank. You can find most of the necessary sizing information on Sheldon's site:

http://www.sheldonbrown.com/bbsize.html

(scroll down and look for Ultegra 6500)
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Old 09-07-11, 06:33 AM   #3
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mbusha is correct that the 6500 is an Octalink crank and takes a V1 Octalink bb. You certainly have an English threaded 68 mm bb shell and, as he further noted, 109.5 mm is for a double crank and 118.5 mm is needed for a triple.
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Old 09-07-11, 06:35 AM   #4
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With modern (external) cranks, these will normally determine what BB is needed, for older cranks, like the 6500; as per mbusha you will need an Octalink, the size can be confirmed by using the Shimano Tech Docs http://techdocs.shimano.com/media/te...9830611829.pdf

For even older (like what would be standard spec of your 90's Trek 1000), it's square taper.

Would be looking first as why your FD isn't working, as changing the crank may make no difference.
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Old 09-07-11, 06:51 AM   #5
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Quote:
Originally Posted by jimc101 View Post
the size can be confirmed by using the Shimano Tech Docs http://techdocs.shimano.com/media/te...9830611829.pdf
For road Octalink cranks there were only two bottom bracket lengths ever made. They are as we mentioned above so there isn't a lot of room for mistakes.
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Old 09-07-11, 09:28 AM   #6
trdsupragt
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Am I correct in believing that I only need to follow what is noted in that document? For a double, which is what I'm looking at, I just need a bottom bracket of these dimensions?

double
Spindle length 109.5 mm
Chain line 43.5 mm
68 mm (1.37 X 24 T.P.I)
70 mm (M36 X 24 T

Man.. I thought bottom brackets were cheaper lol, after seeing mbusha's provided link. All the other ones were, at least. But I'm guessing there aren't very many other "generic" alternatives?

As for my FD, I made mention in another thread topic that I had a Suntour derailleur and crankset, and was told there was a possible chance that the shifter may not like what I've got in the front drivetrain.

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Old 09-07-11, 11:25 AM   #7
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You need an Octalink bottom bracket, BB-5500, in 109.5 mm spindle length and English (1.37x24tpi) threading. The higher line Ultegra version, the BB-6500, is no longer in production so the 5500 is your only current choice unless you can find an NOS 6500 also 109.5 and English threading.

The 70 mm bb (M36x24tpi) is Italian threading and won't fit your bike. Any other 109.5 mm spindle that isn't Octalink is also not the right one.

The Shimano tech document covers all of the variations the road Octalink came in and only one is correct for your bike and crank.
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Old 09-07-11, 11:38 AM   #8
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Sounds about right then. I'm sure a few pop up every once in a while on the bay.

Since I've got a 7 speed drive train, but my current crankset is a 52/42, would I still be okay using my current chain? I suspect the spacing of the chain line may have an effect on this. I think one of the teeth may have broken a little too, or chipped at the very top, rather.

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