UPDATE: I looked over the specs of both cranksets and they do look like they can be changed out! Just double check with someone from Shimano. When you email or call them tell them you have the FC-M521 crankset and wish to replace it with the M665. They will be able to tell you for sure.
Hope this helps ,
Bill
Well I really cannot say that it will but here is the cross reference links....
Here is the link for Specialized Bikes which I would choose which model of the Rockhopper you have..
http://www.specialized.com/us/en/bc/SBCBkModel.jsp?sid=10RockhopperSL&eid=4340&menuItemId=9253
Then search here for the model of crankset on the bike
http://techdocs.shimano.com
Then search for the one you wish to replace it with.
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
Yeah that should work fine, just make sure you have the correct bottom bracket and front derailleur/shifter. If your changing from a triple to a double you might have an issue with the shifter.
Should work, and +1 on checking the bottom bracket.
Adjust the front derailleur limiter screw so it doesn't shift into the 3rd ring position anymore.
And, just a reminder, a double crankset typically has a smaller Q Factor than a triple, and on SOME bikes, this can cause a problem with the crank arms hitting the chainstays.
This might not be a problem with modern cranksets, since so many people switch out their triples for doubles. Just be on the same side.
Good Luck!
Dedicated scholar of bicycles