Join the 2 arrow shafts at the bearing carrier with the coupler shaft inside the
arrow shafts and collars around the outside, same thing at the gearbox, the input
shaft goes inside the arrow shaft, the collar around it.
In all cases, seat the arrow shaft all the way to the shoulder of the inner shaft
and secure with ONE 3X3 setscrew for the time being.
At each collar, pick one setscrew hole and drill, using a #46 drill bit, through the
arrow shaft INTO the shaft inside, creating a divot. Disassemble the joint and
inspect your divot, ensuring it’s deep enough for a setscrew to seat down into it.