Does it already have a 21 in it? Yes? you will need a collet for the FS.40 (8mm shaft) or Kyosho makes a 2 shoe keyed flywheel for the .40 and an exhaust system. You will need the biggest clutchbell and the smallest spur gears available. The .40 tops out at 21,000 rpm's and doesn't like to be run without a heavy load or you will float the valves. Other than that, Have fun and good luck.