I was wondering what kind of engine you have to be running for doubles. I was wanting to put them on the 36" Gravely with a 12hp Kohler. Do you need a special settup or holster for the 2 blades ? My mower has that coupler type thing that is a plate that holds the blade to the spindle. Will the doubles bog down the motor. Will it require more greasing (dosent gravely have sealed bearings in the spindles ? ) Can you do doubles on a 21" walk behind ? is there limits ? What is best combo for sucking the grass up and chopping. The problem I have is the mower cant suck the grass up straight enough to give it that good even cut across it leaves to many straggler peices of grass.