This 2 speed set up fits inside the stock reduction housing, also works with our aluminum reduction housing. No machine work needed. Just send in your old reduction housing and brake lever and we'll send you back everything you need. The 2 speed box will make your transmission an 8 speed with 1/2 gear increments. The 2 speed will speed up your transmission 3.5 gears on the low side and 4 gears in the high side. In other words as example a 20 Tooth gear set would be equal to a 23 1/2 on the low side and 24 tooth on the high side. The kit includes longer top shaft, snap ring to retain top shaft bearing, a 67 Tooth 10 Pitch gear, a 81 Tooth 12 Pitch gear, an inner shift ring, outer shift ring, shift fork, front reduction cover with support bearing for top shaft, special 2 speed pinion with bearing and a 2 speed shift lever. We've tested this 2 speed set up in our "Half Pint" tractors this past NQS season with out failures. See our Youtube video http://youtu.be/tYCWsXQhhBU for installation instructions. We will include a dvd with the instruction video on it.