UKC

When changing from a square bb to Octalink bb will the replacement bb be the same size? I have a new Alivio chainset for my trekking bike but it has an Octalink fit whereas the old one is square. Currently it's a 68   122.5

Thanks.

Yep or there or thereabouts. Octalink bb's are getting harder to get in the right sizes so i'd consider buying another one once you've fitted this one as in a couple of years there may be even harder to get. 

Great, thanks.

you can get charts off Shimano's website that show BB length, chainline, crankset compatibility.

https://productinfo.shimano.com/#/com/?cid=C-432&acid=C-450

To be clear....

"68" means a 68mm width BB shell, this definitely needs to be the same to fit your frame.

The other number is the axle length which depends on the new chainset (and they do vary). You should be able to look it up on the Shimano website as above. Its less of an issue if slightly too long (just need to adjust the front mech outwards to match), too short though and there's a risk of your chainrings rubbing the frame.

Thanks. It was the axle length that I was unsure about. Just wanted to check that Octalink and square placed the chainset in the same alignment.

That's exactly what I needed. Thanks.