I usually have had success threading aluminum tube. Until I tried threading the 3.5" tubes to make the F659 flap rods. They are so short I cannot f8nd anything that will secure them without marring the 5/16" -0.058 tube...
Does anybody sell these already threaded?
Chuck them up in your drill motor.
Drill a hole in a small wooden block the diameter of the rod. Slit one side of the block to the hole so it can be clamped in a vice. This will hold firm with no damage.
The flap rods on the RV-9 (and possibly other models) are hexagonal (part number VA-256), and they are pre drilled. I don't recall if they are pre tapped, but you could probably buy these. I just checked the plans and they call out this part differently from the tubing used in the RV-7. Not sure why (maybe the larger flaps on the -9 require stronger flap rods)?