What bottom bracket (Giant Talon)?

Mr TW has a 2010 Giant Talon 0 which we are using as a turbo bike. The bottom bracket needs replacing. I can’t find the spec anywhere but I’m fairly certain it is Octalink. How do I work out what size? Where would I measure it?

Thanks for any help.

M521?
possibly
https://bike.shimano.com/en-EU/product/component/alivio-t4000/BB-ES300-AK.html

Remove the old one. It should be labelled with the relevant sizes. You need to know the BB width (68 or 73 mm), E-type or regular derailler, plus spindle length. The spindle length is the tricky bit. I think 113 mm is most common, but they vary according to the specific cranks.

I’ve got an ES25 68x113mm, ES52 68x118mm and a ES51 73x121mm.

If you find out its either of those you’re welcome to it for postage. All running fine.

If your lbs is a giant dealer, take the frame number to them, they can punch it into the dealer site and it will tell you and them exactly which one you need.

Thanks all. Took it out and looked. Cheers for the offer Ads but it is a 68×121.