First builds here, two Romanian underfolders & RPK (I ended up with).

After folding the flats and welding in the rails, is it best to install the center support then heat treat or heat treat then instal the center support?
I am heat treating the entire receiver via a kiln (once I sort through the dozens of various opinions on the “proper” way to do it).
I figure that the least I could do before heat treating would be to drill the center support river hole and other river holes as well as dimple them as ream the fire control holes.

Is there anything else that I should hold off on doing until after heat treating?

Thank you for your time, answers, and any and all help.  Keep safe out there.
Raz

Equipment I have:

12&20 Ton hydraulic presses
(AK-B) Rivet Tool
(AK-B) 1mm & 1.5mm Flat Bending dies
(AK-B) bolt cutter style riveter.
Long river tool (I think Toth Tool)
(AK-B) Trunnions rivet hole drilling jig
(AK-B) Trigger riveting jig.
220v Spot Welder
220v Electric Kiln (110v PID)
Media blast cabinet
Parkerizing tank
Spray on epoxy to replicate original Soviet paint
Angle grinder and various disks and wire brushes
Reamers
Drill press

P.S. If there are any builders in the NM area, hit me up.

I wouldn't think there would be any benefit to installing the center support prior to hardening.  On my last couple of bare receivers, which were fully heat treated, I had no warpage, but I was using 1.5mm flats.  1.0mm flats would definitely be more susceptible, as I was able to warp one years ago with a bad spot heat-treat; however, if your receiver is heated consistently throughout and the dunk is vertical(and quick), you will probably have good resultes.  A small amount of warpage is ok, too. Just leave a little bit of excess material to file on the upper rails.  Once you install the center support and trunnions, a small warp should straighten itself out, as I found out with the above-mentioned receiver.

One other thing to consider:  if your center support sleeve is a hardened steel, and is not the same material as your flat, it may not respond to a water/brine dunk in a positive way.  It could soften, which might not be the worst thing in the world, or it could crack, which would definitely be bad.

Either way, good luck!