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  1. #1
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    Old XTR (M952) Crank/BB Problem

    I've got the old Octalink XTR setup on my HT.
    Presently, I have the M950 BB (the fully rebuildable one) with 116mm spindle on the bike with the M952 Crankset.
    It has two 1mm spacers between the drive side bearings and BB shell.
    Everything works perfectly.
    I recently purchased the sealed ctg version (M952) of the BB, also with 116mm spindle. The problem is, if I use the spacers, there won't be enough threads on the drive side bearings to fully engage the BB Shell. Plus, the non-drive side stabilizer piece (the threaded plastic thingie) won't seat on the body of the BB on that side.
    What the heck is going on here?
    Am I missing something very basic?
    To make matters worse, I didn't get an installation guide in the BB box.
    Help!
    Lenny

  2. #2
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    I think Shimano recently took that guide off their techdocs.shimano.com listing. Since you don't mention shell size or chainline, I have pulled out my installation guide for the M952 bb (from when I changed my bb on my M950 crank from the M950 over to M952 bb):

    For a 68mm shell using 116mm spindle the recommendation for a 50mm chainline is to use one 3.5mm spacer on each side of the shell. For a 73mm shell and 116mm spindle use one 1mm spacer on each side of the shell. If you wanted the 47.5 chainline you should have gotten the 112.5mm spindle...
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  3. #3
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    Another dumb question.
    What does 47.5mm or 50mm chain line mean?
    How do I know which I need?
    I measured my BB shell and it's 68mm.
    My front derailleur is an XT, with a 11-34 XT cluster.
    How does anybody even figure this stuff out.
    I took my bike to my mechanic and he put one of the large spacers and 2 small spacers on the drive side of the shell.
    It's all spaced evenly and shifting works perfectly.
    Why would you even need to put a spacer on the non-drive side?
    Any chance you could email me a scan of the installation guide?
    freilich@comcast.net

  4. #4
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    Are you sure the bottom bracket you removed is a116? Could it be a 112.5?

  5. #5
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    Quote Originally Posted by Lenny
    Another dumb question.
    What does 47.5mm or 50mm chain line mean?
    How do I know which I need?
    I measured my BB shell and it's 68mm.
    My front derailleur is an XT, with a 11-34 XT cluster.
    How does anybody even figure this stuff out.
    I took my bike to my mechanic and he put one of the large spacers and 2 small spacers on the drive side of the shell.
    It's all spaced evenly and shifting works perfectly.
    Why would you even need to put a spacer on the non-drive side?
    Any chance you could email me a scan of the installation guide?
    freilich@comcast.net
    For chainline information try this http://sheldonbrown.com/chainline.html

    Sounds like your mechanic was trying to tune your chainline, or just didn't know what he was doing. What is a small or large spacer without a measurement is just guessing. I don't have a scanner but quoted from the guide so you have the essentials for the 116mm spindle. 47.5 to 50mm chainlines are fairly normal in any case.
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  6. #6
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    Pisgah,
    Yeah, the old M950 was marked 116, and when held up to the new M952, the spindle matched exactly.

    Bikinfoolferlife,
    With my chain on the middle ring and middle sprocket in the rear, the line is exactly parallel to the axis of the bike. Isn't that the goal of a proper chainline?
    Measuring from the center of the seat tube to the middle ring is right around 50mm, and like I said, everything works perfectly.
    I think since I mixed components (XT FD, XT Cluster, XTR crank and BB) all the recommended spacer placements are probably out the window.
    I'm sure my mechanic was guessing/using his experience to fine tune the chainline, but in the end, it all works.
    What if anything can be wrong with this method?
    And even if what we did isn't 100% Kosher, the adjustability in the FD derailleur stops makes up for any misplacement of spacers, right.

    And could somebody please tell me what does putting a spacer on the non-drive side do?

    I'm past the point of worrying if the BB is installed correctly.
    Right now, I just want to understand this whole chainline thing!
    Thanks all for answering my questions and let's keep the discussion going.
    Lenny

  7. #7
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    Quote Originally Posted by Lenny
    Pisgah,
    Yeah, the old M950 was marked 116, and when held up to the new M952, the spindle matched exactly.

    Bikinfoolferlife,
    With my chain on the middle ring and middle sprocket in the rear, the line is exactly parallel to the axis of the bike. Isn't that the goal of a proper chainline?
    Measuring from the center of the seat tube to the middle ring is right around 50mm, and like I said, everything works perfectly.
    I think since I mixed components (XT FD, XT Cluster, XTR crank and BB) all the recommended spacer placements are probably out the window.
    I'm sure my mechanic was guessing/using his experience to fine tune the chainline, but in the end, it all works.
    What if anything can be wrong with this method?
    And even if what we did isn't 100% Kosher, the adjustability in the FD derailleur stops makes up for any misplacement of spacers, right.

    And could somebody please tell me what does putting a spacer on the non-drive side do?

    I'm past the point of worrying if the BB is installed correctly.
    Right now, I just want to understand this whole chainline thing!
    Thanks all for answering my questions and let's keep the discussion going.
    Lenny
    First, back to the original problem about sufficient threads. Are the threads in your shell compromised for some reason? Does the drive side bb thread in fine with no spacers?
    In your first post you said there were two 1mm spacers on the drive side, but in the next post you said there's two 1mm spacers and a "large" (is that a 3.5mm ?) spacer, so I'm confused if you were talking M950 on one and M952 on the other.

    Some bb's install with no spacers, depends on the bb's design. In the case of this particular bb (the M952) Shimano recommends the spacer placement as I posted. I don't know why you are questioning the recommendation for spacers on each side of the shell rather than all on one side of the shell; it's simply Shimano's recommendation for installation. If you need to tweak the installation for your particular bike that's something else, although if you have too many spacers stacked on one side then that might explain why you might not have sufficient threads and why your plastic guard doesn't fit.

    I'm not sure what you mean by mix of components, but specific model numbers might help. Is the seattube on your hardtail an oversized one? Is that why you are setup for a 50mm chainline? Your use of an LX or XT or XTR cassette is irrelevant, although I'm curious if it's 8 or 9 speed (if it's 8 speed keep in mind the true center of the cassette is between the 4th and 5th cogs, on a 9 speed it's the 5th cog). \

    Did you ask your mechanic why he did what he did?
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  8. #8
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    OK, I must be an idiot.
    I tried the big 3.5mm spacers on each side.
    Everything works nice now.
    50mm chain line.
    But my biggest question is what chain line do I need?
    If I put the 2.5mm spacers on each side I have a 47.5mm chain line.
    And I'm sure I can adjust the derailleur to shift with that chain line also.
    This is so confusing.
    What's the determining factor on what chainline you need?
    Thanks for all your help Bikinfoolferlife
    Lenny

  9. #9
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    Quote Originally Posted by Lenny
    OK, I must be an idiot.
    I tried the big 3.5mm spacers on each side.
    Everything works nice now.
    50mm chain line.
    But my biggest question is what chain line do I need?
    If I put the 2.5mm spacers on each side I have a 47.5mm chain line.
    And I'm sure I can adjust the derailleur to shift with that chain line also.
    This is so confusing.
    What's the determining factor on what chainline you need?
    Thanks for all your help Bikinfoolferlife
    Lenny
    You need the chainline that works. It's that simple. If you go from 3.5 on ech side to 2.5 then you only reduce your chainline by 1 mm...you need to check your math!

    You think about the small differences too much is all I can say...
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  10. #10
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    You're right, I think about the small stuff too much.
    After all, I've been riding for years with too many spacers on the drive side and none on the other, and it worked fine.
    You should've just smacked me up side the head right up front.
    I'm leaving it as it is and riding.
    It's just funny that bikes will work well with all kinds of mis-adjusted components, and nobody's the wiser most times.
    Thanks again.
    Lenny

  11. #11
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    Quote Originally Posted by Lenny
    You're right, I think about the small stuff too much.
    After all, I've been riding for years with too many spacers on the drive side and none on the other, and it worked fine.
    You should've just smacked me up side the head right up front.
    I'm leaving it as it is and riding.
    It's just funny that bikes will work well with all kinds of mis-adjusted components, and nobody's the wiser most times.
    Thanks again.
    Lenny
    Ride and enjoy. Thinking about the small stuff in perspective is one thing, thinking about the small stuff with no perspective is another...
    "...the people get the government they deserve..."
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