I bought this shaft and cut it down about 1/4 inch or so, but you'll have to find out how much needs to cut for your application. I brought the 8mm outer diamater on the cut side to 5mm exactly like the unmodified side. Then add flat spots somewhere on the 8mm region of the shaft for the spur gear adapter, pinion, whatever. You can figure out how to space it based on your motor and pinion. The other flat spot is on the 5mm end that I used a lathe for for the outdrive and set screw.