[Av8torTom]

Just being cautious here before I start drilling holes and countersinking my RV-9 spar.

There are two bags of tank attach hardware - 1922-1 and 1922. I assume the 1922-1 platenuts and screws are for the top and bottom spar flange as seen in the photo below:

Then I enlarge the platenut rivet holes, machine countersink #40, rivet platenut under the spar flang, then machine countersink screw hole #30 on top of the flange???

Sound right?

 

[N427EF]

I have not worked on a 9 but I believe the detail for plate nut attachment
are the same on all models.
The sequence you describe is the way Vans has you install the plate nuts.
The reason you rivet the plate nut into place is so you'll have a hard center
(plate nut) to hold the countersink pilot in place.
The material thickness of the spar flange is too thin to hold the entire depth of the countersink hence a countersunk nut plate to absorb the dimpled skin of the Tank or nose skin.
I remember using clecos from the bottom to hold the plate nuts in place
for countersinking, this gave me a chance to clean up the hole from the backside before riveting the plate nuts.