Hi All –
I cut the hole in my firewall for my conversion. I am going to attach the new steel access plate using screws on the interior, and then welding in some captured nuts on the engine compartment side of the firewall. Being that each nut will just get a quick zap of a couple of spot welds, do I need to worry about what finish is on the nuts (raw vs zinc, etc)?

Thanks!

You could always rivet some nut plates instead of welding.

for your application probably no worry if plated or not with a good mig welder but you could always wire wheel or balst them clean metal too. be very care full of gas fumes in the tunnel if you weld near there.

a nice clean alternate way is to use rivnuts or Nutcerts these are trade names of a captive nut, you can even install them in blind panel (no rear acess) ionstalls kind of like a rivet.

nutcerts or similar are the way to go - no burned paint, just as good for this application I would think and less "invasive"

I'd take a quick grind to the edge's your going to weld...

Dont need much, just 1/2 a sec on each edge.

remove the zinc plating before welding. It's going to burn off anyway, and the zinc will make it harder to get a good weld. Also, used flanged nuts for increased weld area.

Oh! Good idea!
I'll do that on my hitch plate.

The nut inserts work great. Weld nuts are better but more involved. The fold over tab nuts also work without as much precision.

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http://www.aircraftspruce.com/catalog/hapages/tinnerman.php

Another trick is to drill a hole about four sizes smaller than the tap you want to use, and then use a tapered punch to get the hole close to the correct drill size (to bugle the hole) and then tap it for the size screw you want to use. This takes more time, and works best if you have someone back up the hole with a improvised mandrel.

If you have an air powered rivet gun (or the muscles to wrestle a hand riveter) threaded rivets would be a really clean and nice solution for this.

I always put a bolt thru the nut before I weld the nut to the plate. I think it acts as a heat shrink to keep the expansion of the steel minimized.