Hi
Can anyone tell me what size BB do I need to upgrade the square taper bb on my 2015 rockhopper sport an external one? Cheers

Surely that depends on which chainset you are going to fit?

Ah, ok, Im new to this. Cheers

If you currently have a tapered bb you will need to take it to a LBS and have a tread reamed into the bb shell. You can then fit a standard external bb

Know idea what that is on about^^ You can’t ream a thread and the thread is the same for Sq taper and external BB’s…

But If you want to use external BB you will need to change the cranks, for example if you buy a shimano chainset it will come with the BB.

Sorry for the crap advice/ explanation ( I’m on night shift ). By ream I meant clear the threads out. I’ve always thought it was good practice to have this done when changing to a different bearing standard so that it all fits nice.

Can’t say I see the point. Do you run a tap through every tapped hole when you fit a different bolt?

OP – you would be looking a minimum of £50 for entry level cranks (Shimano Deore) and then £10 for an external bottom bracket + fitting. Those are chain reaction prices so LBS will most likely be more.

Why do you think you need to upgrade the bottom bracket to a less reliable external one? It will be lighter and slighty stiffer with a HTII style set of cranks and BB but just wondering if you really need to or if it’s just something being done on a whim because you’ve seen it on higher spec bikes?

So I assume you’ll be getting new shimano cranks as well. In which case there’s just one BB size to get and you use shims/washers included with the BB to get the correct chain line. So measure the gap between your inner chain ring and frame currently and use that as a guide. Or take it to an LBS 🙂