Instructables
Picture of How To Change The Rotors On Disk Brakes
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My bike was recently tuned however it developed an irritating screech and lack of braking power, so we decided to change the disks and fix the problem. Changing the disks is vary easy all it requires is a torks T25 screwdriver, and preferably a torque wrench. If you don't have a torque wrench don't worry unless you are racing, screw tight will do. 
 
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Step 3: Change Disks

Any torque screwdriver will do to undo the screws. Undo the screws in pairs, first top right then bottom left exc..
Once the disk is off place the new one in place and first hand tighten all screws in the same pattern.  Then if you have a torque wrench correctly set it to 40-60in lbs. To set the wrench start by un doing the wrench completely and then screwing the handle up to somewhere between 40-60 in lbs (not ft lbs). If you don't have a torque wrench (or it can't go to 60 in lbs) do your best to tighten all the screws evenly. Once all the screws are done up the wheel can be placed back on the fork and the quick release can be tightened. Then take the bike for a test run and make sure the brakes are working, trust me finding out at the top of the hill is  much worse. 

Sorry about the super wide angle pictures.