yugyug

I have an 80s 6spd road bike with an older type unsealed bottom bracket (shell width of 68mm, english) which is starting to die, and I want to replace it with a Shimano sealed BB-UN55. The current axle length seems to be about 125mm, but the 68mm versions of the UN55 only comes in axle lengths of 122.5 or 127 (sometimes listed as 127.5).

2.5mm doesn't seem like much, and it seems to me that I will just need to adjust the front derailler a bit to make it play nicely with the drivetrain. Is that correct or is there something I'm missing? Any difference with going up or down in length?

hueyhoolihan

get one of these, in the size you need and put it on the drive side.

or you can get them here in bags of 20 and singly in various sizes and prices.

it's also possible that you could buy either 122.5 or 127/127.5 BB and not have much of a problem.

BTW, if the rear hub is a freewheel rather than a cassette hub, you can use the same spacer on the hub before threading on the freewheel, essentially moving the freewheel out the width of the spacer. that may help, could possibly cause clearance problems with chain and dropout though. it will be evident soon enough.

if longer or shorter were the only two options, i would pick shorter. i find it less objectionable to have a poor chainline on higher gears, it seems to make less noise.

3alarmer 

..Google Chainline and Q factor with regard to bicycle, but in essence, you'll probably do OK.

On something that was set up pretty close in the original, it's possible to end up with your inner chainring interfering with the stay on that side, if you go too short.
But generally, the only thing you affect is the chainline, which is not all that critical, and might even be improved...depending on how it is now.

Bill Kapaun

Many of the older spindles have a noticeable offset on the DS, so measuring the overall length doesn't give you the correct info.
Is there a "code" on the spindle such as 3-P, 3-S or similar?

There's a good chance you can't find a cartridge BB that is long enough.

nfmisso

The codes Bill referred to are listed in a table about 20% of the way down this page: Sheldon Brown's Bottom Bracket Size Database along with the equivalent modern symmetrical sizes.

yugyug

Thanks everybody thats excellent info, I have some solutions and stuff to check.

Thanks I found those codes on Sheldon's site, will check. I think I know what you mean, but what does DS stand for?

Bill Kapaun

Drive Side