Square Taper BB
 

HI,

i replaced my square taper BB on my MTB around a year ago and kept the drivetrain as it is. i have now purchased a new triple crank, cassette and chain without really thinking about my bottom bracket. looking at the specs for the new crank, it recommends a 68x122mm. my current BB is a 68x127mm. i remember it was difficult to install the BB and im worried that i may ruin the threads taking it out and putting another back in. will the extra 5mm make too much difference?

i guess i could just fit it and see if i can adjust my front derailer.

any comments would be great!

Thanks

Phil

The extra 5mm is in the length of the spindle, it has nothing to do with the mounting threads. The only downside to using your old BB, aside from a slightly "off"chain line, is the time it takes to try it.

Thanks for the reply, i know the difference would be in spindle length but was wondering what the impact would be throwing the chainline off slightly... when i installed the last BB the threads within the frame were slightly damaged and it made it difficult aligning it and installing it.

Like you say to fit it and try it. might just get a new BB and go for it

Hold your fingers 2/10ths of an inch apart. That's 5 MM. It's not likely to make much difference at all, especially with a triple.

If you are going from a double to a triple, are you going to have an FD and/or shifter issue?

If your threads are boogered up, don't force it. Take it to a shop and have them chase the threads to clean them up.

The extra spindle length will matter most on the ends of the cassette, with very little difference in the "normal" riding range. The bigger issue will be in front and if your FD has enough range within the limit screws to reach all the chain rings. That would be what I check first.

As long as the crank isn't moved so far outboard that the front derailleur can't reach the outer ring, it should be ok. And the way to determine this is to mount the crank and see what happens.

I hope the OP used either grease or anti seize compound when installing the replacement BB. The 5mm of OVERALL length is actually 2.5mm of chainline change (if the numbers are the real in suto ones). That's less then 1/8", not much for a frt der to handle vis small limit screw and cable tension adjustments, unless things were already at their limit of cage travel range.

Thing I do when replacing this stuff is to check actual pre service chainline (it's not uncommon to find slight "offness" from the factory), the frame clearance and how much change will work with this detail and the frt der's ability to adjust either more inward or outward. Andy

Thanks for the info much appreciated! It's a triple at the moment so no derailleur issues. Used plenty of grease installing the old one just didn't like how it was fitting and how hard I had to tighten it (don't have a torque wrench :()

I've played with the limit screws and there is a bit of room to move around on the largest and smallest cassette rings and similarly in the front chain rings. Gonna give it a go and see if I can get it shifting nicely and if not go for the BB replacement.

Thanks again and will post the results when the parts show up!