[Closed] what bb do i need for this crankset?

A Truvativ GXP.

just googled it, will any external bb fit this crank ?

No, must be a truvative GPX one.
[url= http://www.merlincycles.co.uk/Bike+Shop/MTB++Parts/Transmission++Gears/Bottom+Brackets/SRAM++Truvativ+GXP+Bottom+Bracket_2090.htm ]BB[/url]

Any GXP one yes, not a Shimano one without adapters - the axles are wider.

Think you're stuck with GXP bottom bracket, other external bb's are interchangeable but not the truvativ ones.

I fitted a Hope one to my Truvativ Stylo GXP but you will need the little adaptor spacer thingies that Hope sell. I'm guessing if you can do this with Hope that you can do the same with Shimano or whatever as long as you have the spacer kit. Lots on e bay.

Hope spacers won't fit others, they clamp tight around the bearing so plastic top hats etc will interfere with them and get damaged.