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Does anyone know if Dura Ace external (10 speed) bottom bracket cups are compatible with XT M760 series cranksets?
 

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I'd check the shimano site, but off the top of my head I would say that they would be too narrow. Road stuff is designed around a 68 - 70 mm BB width whereas MTB stuff is designed for 73 mm BB (or 68 with spacers).
 

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no they are not

thecrazyfinn said:
Yes they are. Spacers may need adjusting if you have a 73mm BB shell.
these BB cups are not the same. they will not work on MTB bike. thats way they say road or mountain on the cups. so that you can tell which is which.
you cant use them and have your chain line correct.
 

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Dover said:
these BB cups are not the same. they will not work on MTB bike. thats way they say road or mountain on the cups. so that you can tell which is which.
you cant use them and have your chain line correct.
Chainline is determined by the spindle, which is integrated into the drive-side crank arm on external-bearing cranks (with the exception of the Truvativ external-bearing BB's with ISIS-style spindle).

The cups are compatible as long as they'll fit your BB shell. But road BB's lack the extra seals of a MTB BB, which is a major issue for external-bearing BB's, which are already more open to the elements.
 

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not true

thecrazyfinn said:
Chainline is determined by the spindle, which is integrated into the drive-side crank arm on external-bearing cranks (with the exception of the Truvativ external-bearing BB's with ISIS-style spindle).

The cups are compatible as long as they'll fit your BB shell. But road BB's lack the extra seals of a MTB BB, which is a major issue for external-bearing BB's, which are already more open to the elements.
Chainlines are not determined be the spindle. They are determined by the size of the BB cup, spacers used, and the way the crank was engineered. Shimano cranks are not design to have an adjustable chainline. The chainline is set at 50mm for all external BB MTB cranks. (hollowtech II)
Road cranks are designed to have a 42.5mm chainline and are not design to use spacers at all. The cups are not the same size. If they were interchangable, Shimano would not have specified the difference if it was not important. The BB cups have "Road" or Mountain" writen on them for this reason.
There is also no difference in the seals between Road and Mountain BBs. I've had many of both types apart and they are exactly the same inside. there is only a difference in the actual size of the cups not the bearing inside.
 

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Dover said:
Chainlines are not determined be the spindle. They are determined by the size of the BB cup, spacers used, and the way the crank was engineered. Shimano cranks are not design to have an adjustable chainline. The chainline is set at 50mm for all external BB MTB cranks. (hollowtech II)
Road cranks are designed to have a 42.5mm chainline and are not design to use spacers at all. The cups are not the same size. If they were interchangable, Shimano would not have specified the difference if it was not important. The BB cups have "Road" or Mountain" writen on them for this reason.
There is also no difference in the seals between Road and Mountain BBs. I've had many of both types apart and they are exactly the same inside. there is only a difference in the actual size of the cups not the bearing inside.
Interesting, but entirely incorrect as to chainline. Chainline is determined by the spindle, which is what determines how far apart the cranks are (as cranks & BB are designed for the cranks to be centred on the bike), this is true for all cranks, not just external BB cranks. The spacers are there to allow the same BB to work in 68 and 73mm BB shells (with the effect of allowing some minor tweaking of the chainline as well). And the cup size is only necessary to take up space between the crank and the BB shell as the spindle is no longer held inside the BB cups. The bearing race sizing I'm not sure of, having not measured it, but Shimano has often in the past specified a non-existent division between road and mountain parts.
 

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This is the Q-factor

thecrazyfinn said:
Interesting, but entirely incorrect as to chainline. Chainline is determined by the spindle, which is what determines how far apart the cranks are (as cranks & BB are designed for the cranks to be centred on the bike), this is true for all cranks, not just external BB cranks.
what you are talking about is called the "Q-Factor" not "Chainline".
it does not matter what length the spidle is when you are determining chainline. The right side crank arm is bottomed out against the BB cup. the spindle could be 10 ft long and your chainline would still be the same.

The spacers are there to allow the same BB to work in 68 and 73mm BB shells (with the effect of allowing some minor tweaking of the chainline as well).

This is not true either. Shimano left (non-drive) BB cups are not designed to be able to take 2 spacers and you run the rist of destroying your BB shell threads. take a look, it is way shorter than the drive side threads.
73mm BB = 1 spacer on the drive side.
68mm BB = 2 spacers on the drive side, 1 spacer on the non-drive side.
 

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Spacers

You can run spacers outside the BB cups/bearings to take up the extra space.

It can be done with Shimano 'road' cups. Perhaps not ideal, but it is possible.

Uhhh, and in this case q-factor and chainline are directly related. For practicle purpouses you cannot adjust one with out effecting the other. Sure you could have a 10' spindle and slide it all the way to one side :rolleyes:

The cranks are designed to be centered so moving the chainline in to say 47.5mm (requiring spindle modification) will also reduce the q-factor. Of course you could just slide it one way a bit but then you'll spend the rest of your life at the chiropractor, or other specialist of your choosing.
 

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Shayne said:
You can run spacers outside the BB cups/bearings to take up the extra space.

It can be done with Shimano 'road' cups. Perhaps not ideal, but it is possible.

Uhhh, and in this case q-factor and chainline are directly related. For practicle purpouses you cannot adjust one with out effecting the other. Sure you could have a 10' spindle and slide it all the way to one side :rolleyes:

The cranks are designed to be centered so moving the chainline in to say 47.5mm (requiring spindle modification) will also reduce the q-factor. Of course you could just slide it one way a bit but then you'll spend the rest of your life at the chiropractor, or other specialist of your choosing.
Odd that I'm agreeing with you here(since we seem to have a mild debate ongoing elsewhere), but you're entirely correct.

Q-factor is the width of the crank at the pedals, chainline is half the width at the middle ring (or the midpoint between the rings on a double) for a centred crank (to be exact, it's the distance between the middle of the seat tube and the aforementioned measuring point at the chainrings). Both are controlled by the spindle size and crank design, not the BB cups, although one can tweak chainline by setting the BB cups so the crank isn't centred.

You can run an extra spacer on the non-drive side of the BB because this side sees significantly less stress (which is one reason why the NDS threads are shallower).

And also there's the fact that running a 50mm chainline is a great way to ensure that shifting is non-optimal, particularly on the big ring of a triple and the middle-high side of the cassette. The middle ring should be centred with the 5th cog of a 9spd cassette for optimal shifting. Shimano has chosen to give up optimal chainline in order to run external bearings to make up for the mistake of not going to a larger diameter BB shell when it became obvious that the larger spindle size of Octalink had a detrimental effect on bearing life due to the use of undersized bearings to fit the standard BB shells.
 

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Shayne said:
You can run spacers outside the BB cups/bearings to take up the extra space.

It can be done with Shimano 'road' cups. Perhaps not ideal, but it is possible.

Uhhh, and in this case q-factor and chainline are directly related. For practicle purpouses you cannot adjust one with out effecting the other. Sure you could have a 10' spindle and slide it all the way to one side :rolleyes:

The cranks are designed to be centered so moving the chainline in to say 47.5mm (requiring spindle modification) will also reduce the q-factor. Of course you could just slide it one way a bit but then you'll spend the rest of your life at the chiropractor, or other specialist of your choosing.
where did you get your engineering degree???
you seam to think that the engineers at shimano has aren't as smart as you. if it was designed to be adjustable there would be spacers and instruction to do this.

I'm sure after you've done all of this f'n around with the chainline and q-factor you'll have some **** ass front shifting and then start complaining about that.
why not just run the correct BB with the correct spacers?
 

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Dover said:
where did you get your engineering degree???
you seam to think that the engineers at shimano has aren't as smart as you. if it was designed to be adjustable there would be spacers and instruction to do this.

I'm sure after you've done all of this f'n around with the chainline and q-factor you'll have some **** ass front shifting and then start complaining about that.
why not just run the correct BB with the correct spacers?
Funny, but there are spacers. Might want to think about why they're there. And bringing the chainline in to 47.5mm will improve shifting over a 50mm chainline (which is why Shimano spec'd 47.5mm in the first place, only going to 50mm when outside bearing BB's & suspension frames made the proper chainline impractical on some bikes)
 

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you can do it!

So, is it possible? Absolutely! A dura ace external bb can be used w a mtb external bb crankset. Why would you do it? Probably because you have a road bb sitting around. The dura ace cups are about 1.5mm thicker, so you use 1 less spacer. 68mm = 1 on each side, 73mm=0 spacers. I have done this twice with complete success, once on a turner rfx, and once on a salsa fargo. It brings the drive side crank in, but less than 1mm, so its no problemo. It may even be a good thing. I have also heard that the dura ace bb is the only one that uses stainless bearings....:eek:
 
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