changing out the bottom bracket..
I am thinking of upgrading my cranks, and the current ones are square taper, so I'll need either an ISIS or Octalink depending on what crank I want to get.
The bike i have is a 2006 Marin Northside Trail
What specifications do I need to measure to know which one to get?
If you stick with.....
an ISIS splined crankset you shouldn't have to measure anything. Any ISIS crank will mate perfectly with your current bottom bracket. If you switch to an Octalink set up, you'll need to know your bottom bracket shell width, and what spindle length will give you the correct chain line for the bike and the crankset that you chose. However is you're up for it I would recommend and upgrade to a newer two piece external bearing system like the Shimano Ocatalink II or the Race Face X type. The system is noticeably stiffer than the old Ocatalink or the ISIS type interfaces, and is easier to service and maintain. The crank and bottom bracket are intergrated so you don't have to shop for "the right size", and they are compatable with both 68 and 73mm bottom bracket shells. The only thing you'd have to measure is the chain line of your current cranks. This is easily done, measure from the center of the seat tube to the middle ring of the crank. If it's between 48 and 50mm you should be good to go with an Octalink II or X type crankset.
"I do whatever my Rice Cripsies tell me to!"
My bad, my BB is actually powerspline, so that is also ISIS? So Any ISIS should work with it?
I thought that there were different types of ISIS BB's, but if not, hooray!