[Tom Ore]

If you look at the four shafts in the gearbox, there is a 10 mm center distance between the first three with a total of 40 teeth in the mesh. The first pair of shafts have a 10T / 30T combination, the second pair has a 15T / 25T combination. All three of these shafts have a 12.5 mm center distance to the output shaft. The final reduction is a 10T / 40T combination or a total of 50 teeth in the mesh. Since all three shafts are 12.5 mm from the output, the final reduction gear can be placed on any of the first three shafts and still mate properly with the output gear. To make the 10.4:1 ratio, attach the output gear directly to the input gear. This leaves a stack of 4 gears with only the first and last actually being used. You have the 26T helical input gear, the 10T gear, a 25T gear, and the 10T output gear. The transmission ratio is then 26/10*40/10 = 10.4:1. The advantage of this arrangement is that the only modification required is to attach the input gear and output gears together. The gear box and spacers do not need any modification - the spacers just need to be moved around a bit. The output gear has a very thin spacer under it - we moved that from the third shaft from to the first.