How To Fabricate A Custom Shaved Firewall With Sheet Metal

Beyond The Iron Curtain

Whether you are looking to make a statement or just want to clean up underneath the hood, a smoothed firewall will always be near the top of the list. From the factory, most firewalls are a utilitarian component of the vehicle; it is there to stop fumes and heat from getting into the passenger compartment and to hang a few accessories on, and that is about it. For hot rodders, the firewall presents yet another opportunity to customize our cars. There are many ways to go about it, from simply shaving unnecessary holes, to complete replacement..

The most basic smoothing typically involves relocating or removing bits and pieces you don’t need or want, shaving the holes, and sometimes shaving the big A/C or heater box. This task is about as simple as shaving a door handle or side trim. Replacing a firewall, however, is a more monumental task. If this is your goal, you have your work cut out for you (pun intended). As there is not much on the market in the way of pre-fabbed firewalls, you will need to build it yourself. The trick is to plan ahead.

Starting from scratch means everything must be created. The most important pieces you will fabricate are the brake master cylinder mount and the lower steering column mount. If your original window wiper motor was firewall-mounted, as most are, you must take this into consideration. In the case of the Red Dirt Rodz-built 1969 Plymouth Road Runner shown here, the windshield wiper motor was the main reason for gutting the original design.

Mopars, as classic and aggressive as they are, are not known for spectacular build quality from the factory. The firewall is full of wrinkles, the wiper motor is both sunk in and juts out at the same time, and it just doesn’t look good or fit within the parameters of a high-end build. We could have tried to shrink the wrinkles and smooth it out, but in the end, the best solution was to cut it out and start over.

While the overall scope of this project is pretty big, the process is really quite simple, and the tools needed to do the job are likely already in your garage or shop. The major tools required are a welder, sheetmetal cutting tools (Sawzall, electric grinder with cutting wheel and flap disc, and a plasma torch, if available), clamps, and the usual assorted hand tools. Don’t forget nerves of steel, because this is major surgery on your car. Keep in mind, once you make that first cut, you are committed to finishing the task.

Most cars of this era use 14 gauge steel for the firewall, so we stayed with it because 14-gauge is considerably harder to work with than the more common 18 gauge materials used on floor pans and exterior panels. It is harder to bend, which makes it better for the job as a firewall, but that also means that any required corrugations or bubbled sections will be harder to create. For the Road Runner, we opted for a weld-in bubble we created from more 14 gauge for the brake master. This allowed us to keep the factory pedal bracing, plus it gives the new, smooth firewall a little character beyond a flat panel.

Everything else that is normally mounted to the outside of the firewall was moved inside. The A/C mounts were moved to the dash and cowl, and the windshield wiper motor was replaced with a street rod unit (specifically for a 1932 Ford truck) and studs were welded to the cowl and dash bracing. The rest is just a matter of making patterns, tracing, cutting and welding.

The biggest trick of this project is to start with the firewall in the car, create the new firewall, and then transfer any holes or specific marks to the new piece before cutting out the old one. This helps keep everything where it is supposed to be when matching up immovable structures, such as the pedal braces. Take a lot of measurements and make small cuts instead of big ones.

When it is time to weld, take your time and go slow. A new, smooth firewall that is warped from too much heat is going to take a lot of filler to get flat again. Before you drill any holes into that new firewall, be sure they are in the right place, or risk having a mess on your hands.

Creating the new firewall for the Road Runner took the crew at Red Dirt Rodz in Stillwater, OK about a week to complete, from cut to prime, with the major work taking three full days. Following the suggestions outlined here, and injecting some extra effort and creativity, you can also transform that funky firewall in your classic ride using a minimalist approach with stellar results.