Cartridge-type Bottom Bracket Sizing

XmisterIS

Purveyor of fine nonsense
The bottom bracket in my MTB has finally bitten the dust - bearings are shot to buggery. It's a 68mm Truvative Powerspline (ISIS octalink bottom bracket).

Now, I know I need to get another 68mm one or it won't fit ... but what about the axle measurement? Can I choose the size I want to put the cranks as close together as the frame will allow, or as far apart as I like?

And can you mix a Shimano octalink BB with Truvative cranks?
 
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