I was fortunate to have a couple of pristine '36 Chevy pickups to look at while doing my restoration and also a great VCCA technical advisor, Jim Payne, who provided detailed information. He even had a collection of GM Service Bulletins pertaining to the '36 PU. All of that showed that when the '36 PU left the factory the firewall holes consisted of:
10 1/4" holes for the wire loom brackets and the elongate 4-hole (oil pressure, temperature, throttle cable, choke cable) grommet and rear hood latches.
4 3/32" holes for the VIN plate.
3 larger holes for the vacuum line, dashboard positive electric feed wire from the starter motor switch post and electrolock cable.
All of those holes, along with about 40 others were in my firewall when I got the '36. Now only the 17 original holes are there.
Jim Payne's library of '36 Chevy information was so complete that it included a Service Bulletin on an optional fan shroud that is so rare that in 55 years of owning my '36 and being acutely interested in the '36 Chevy trucks the factory fan shroud on my own '36 is the only one I've ever seen. It took about 40 years of searching to find this one.
I use a Mig welder for my welding, used .023" solid wire for the sheet metal work, it helps to minimize distortion and I think it's easier than oxy-acetylene welding. The measles are still there, I haven't started body working the firewall yet. My plan is to complete all of the metal work on the vehicle before I start body working any panels. Just my preference.
I use nothing but 3M Panel Bond on cut to size patches. No heat, no warp, and done in an hour. Just rough up with 36 grit around holes and the same on the patch. Smear enough on to squish out around the edges and the patch is waterproof. All airliners and super sonic fighters are panel bonded so an AD pickup shouldn't have a problem.
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Evan Doesn't panel bond need an overlap? I've never used it, but I would think that patching little holes that the OP is talking about would either require a backer or something to support the patch level with the original metal. Of course on a firewall, the backers would be covered up by the blanket or floor covering. Do you have any pics or an explanation of the process?
The "after" photos do look nice.
Kevin First car '29 Ford Special Coupe #2 - '29 Ford pickup restored from the ground up. Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com] Busting rust since the mid-60's
The firewall patches are about a half inch bigger than the holes and the top ranges from 3/4 to 1 1/2 inch overlap. Picture of truck is recent on what was done 16 years ago and Model A was done 5 years ago; zero problems with either.
"I use a Mig welder for my welding, used .023" solid wire for the sheet metal work, it helps to minimize distortion and I think it's easier than oxy-acetylene welding."
You make an excellent point. Oxy-acetylene welding does put a lot of heat into the workpiece, and with sheet metal distortion results. My firewall repair was done so long ago that Mig may have been around, but it was not in widespread use and I had not even heard of it then.
Because I had taken 3 semesters of evening adult education classes in auto body repair I knew how to shrink sheet metal and that allowed me to get the firewall flat after all that welding. Metal shrinking was also required on the bed of my '36 PU that (no exaggeration) looked like a sack full of rocks because it was so lumpy.
Just for fun I looked at what Panel Bond is and it seems to be glue for sticking on patches. Being old school, I prefer an actual welded in patch in body sheet metal. When the welds are ground smooth what remains is a solid panel exactly like what was there before the hole was made. I do understand the appeal of a product that is quick and easy to apply, not requiring any metal working skill. It's the same appeal that Bondo has.
Panel adhesive is not Bondo. Not even close. It is what I use on the pinch weld seem at the belt line of the cab. replace rusted out spots, media blast, epoxy prime, fill seam with panel adhesive, and sand to butter smooth.
Ray W (Yar), If you have owned a Chevy pickup anywhere after the mid 90's you have experience with panel bond. The door hinges are bonded to the body---no weld no bolts. Haven't noticed a lot of Chevy pickup doors that have fallen off and are laying on the roadway which is pretty good for Bondo. Might be best to watch for parts falling off an F22 or F35 JSF fighter since they are "Bondo'd" together. I will go back to rivets and weld right after I move from my house back into a cave.