I have put a double crank on a bike that originally was intended for a cheap stamped triple. i used a square taper FSA compact double, and I had to add chainring bolt spacers so the nominally 10 speed crank worked well with a 7/8 speed chain. I also had to figure out a different BB size to get the optimal chain line. when I was done. I used the 50T ring for all 7 rear gears, and the 34 just with the 3-4 lowest(largest) sprockets as a hill climber.