The instruction video says to use two thick washers on the tail drive shaft cross bolts. However, a thick washer under the head of the bolt places the shank to thread transition area of the bolt right in the shear joint between the tube and bearing/coupler rather than letting the bolt shank pass completely through the hole.
Since it's better to have the bolt shank pass completely through the material, I decided not to use a washer under the bolt head. Because the drive shaft turns at 3000 RPM there is some consideration for balance so I decided to use a low profile nut with a thin washer to better match the mass of the bolt head on the other side of this spinning tube. This, however, left a lot of threads exposed and working against a good balance so I shortened the 12 tail drive shaft cross bolts by about 1/8" leaving 2-3 threads past the top of the nut and a fairly well balanced bolt.
