Hi, I was wondering if someone could help me out please, the LBS is closed for Easter!

I had my bottom bracket replaced at the LBS last week and requested a 123mm BB as the Shimano techdocs (https://techdocs.shimano.com/media/te...9830621467.pdf) stated this would be compatible with my crankset and Front Derailleur (FC M311 and FD M311).

However, now the front derailleur will not reach out as far as the large chainring when pushed manually. My front chainline now seems to be about 52mm.

I therefore have 2 questions;

1) Does this mean the BB length is too big?

2) if so, why did the techdocs specify a 123mm?

I can't blame the LBS and it doesn't seem that I'm at fault here either but replacing the BB again seems annoyingly likely.

Thanks very much.

A few things to think about. If you told the LBS, "use this BB," it's mainly on you if it's wrong, unless they installed it wrong. OTOH, I do think they should have checked chainline and asked you of that number (which is too big if you measured it right and you are setting up a road bike) is ok with you. So, double-check a few things:

1. Do you have a double or triple?
2. How did you measure the chainline?
3. Did you torque the crank fixing bolt to spec?
4. Any warps in the chainrings?
5. Many BBs come today with a spacer or two. Could the LBS have used them when they should not have done so?

Did you try adjusting the 'H' limit screw ? Or is it still set for the old BB ?

The first question is how much room do you have between the chainrings and chainstay. Ideal is 2-3mm, and if you have more than that the spindle is longer than you need (as the high chainline indicates).

Also understand that not all 123mm spindles are the same. Some are fairly symmetrical, some have more offset to the right. So while the overall length is right, you're specific spindle might not be.

Now 52mm chainline isn't that far off, so as asked earlier, have you backed the high gear (outer) limit all the way?

If so, and if you know how to install a BB (and have the tools), you might try reversing the spindle, so the shorter offset is to the right.

Are you positive they didn't install a 127 and call it "close enough"?

Thanks for the quick replies.

Road Fan, in order:

1. I have a triple, Shimanno FC-M311 48-38-28. The FD is also an M311 so there should be no compatibility issue there.
2. I measured from the middle chainring to the middle of the seat post
3. No, just with reasonable force but I don't think this is the problem as the FD seems to still need quite a bit of distance in order to shift the chain and clear it. At present the front plate reaches as far as the big ring.
4. No warps that I can see.
5. I'll need to check that, can I tell without removing the crankset?

Homebrew, I've taken the limit stops out entirely to check how far it would swing.

I bought the FD second hand, it could be knackered?

Or maybe the BB spindle has unusual tapering, it was a generic one they fitted, the techdocs stated a UN26.


FBinNY, where do I take that measurement from?


Bill, I didn't watch them do it but I can only assume they fit a 123 as I requested.


Couple more things: I changed my FD which did shift into the big ring using the old BB (just about) and replaced it with the FD-M311 which I got second hand so is there a possibility it's broken? I think I threw the old one out as well which I realise doesn't help matters. Oops.

Thanks again for all your help

Have you verified that you actually have the crank you specified?
My Globe had a different low end Shimano crank then in the specs, which used a different length BB.

Can you call the LBS and see if they remember if they installed the same size BB that they removed?
It might be a situation where they did what you told them, not what you really wanted.