Modern Rodding TECH

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Fabricator welding new floorpan in place
1. As with any sheetmetal project patience is your friend when welding. Select the correct temperature, welding rod, and be patient and it will turn out much better in the end.
Sheetmetal Surgery
Pro Tips for a Floorpan Repair on a Perforated Pontiac A-Body
By Barry Kluczyk Photography By THE AUTHOR
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hether it came from the bone-dry desert or the heart of the Rust Belt, corrosion is the bane of every piece of vintage metal. Even if it’s not immediately visible in the quarter-panels or rockers, there’s a good chance water found its way to the crevices deep inside the body, in the toeboard panels at the front of the floor, in the plenum area in the cowl, and, very likely, beneath the trunk mat, turning the floor into sheetmetal Swiss cheese.

Sure, there are exceptions to the rule, and we’ve all been amazed to discover rot-free steel, but if you or your preferred resto shop is planning a thorough reconditioning of your favorite chunk of Detroit iron, a measure of rust repair will more than likely figure into the project.

There was no question the ’64 Pontiac LeMans convertible (GTO same floorpan) project car in our story would need a date with a sheetmetal surgeon. A Midwesterner all its life, the A-body suffered from typical corrosion from stem to stern. Previous patches kept the car on the road longer but couldn’t stave off the inevitable.

“You never know the full extent of the rust issue until you get the body stripped and media-blasted, but even before that we knew this one was going to need some serious attention,” says longtime and recently retired restorer Nyle Wing. “The car had been in the owner’s family for decades and he told us about some of the previous repairs, but even we were surprised to see rust holes literally from one end of the car to the other.”

The extent of rot and patchwork throughout the Pontiac’s body was extensive and would ultimately take weeks of professional attention, but we caught up with the project to zero in on one of the more common repairs that DIY enthusiasts will attempt: floorpan replacement.

It’s a project that must be performed accurately to ensure a strong, long-lasting repair. We followed along as the Pontiac’s cancerous steel was sliced out and the replacement metal grafted in with surgical precision. The project also demonstrated the more time-consuming butt-weld method rather than lap-welding, a technique that makes all the difference in a high-quality repair, especially when it comes to a show-quality final finish.

“The butt weld is the way to go for a strong, factory-type repair,” Wing says. “It takes a little more time, patience, and experience, but the end result is worth it.”

It also begs an important question: Is a floorpan replacement a DIY project? Well, the welding isn’t particularly complicated, even for those with only moderate experience, but the job requires additional tools that might not be in everyone’s toolbox, including a spot weld cutter for a drill, long body panel vise clamps, and a body hammer. An air hammer or plasma cutter speeds things along, too, as seen in our story.

So, yes, it’s a project that could be tackled at home, but the chance for warping the comparatively thin sheetmetal used in floorpans is high, particularly with butt-welding. With a professional repair, the bulk of the cost will be in the labor hours, as the cost of the replacement metal is comparatively inexpensive.

For the project illustrated in our story, only a front floor patch panel was used, which can typically be found for less than $100 for GM A-bodies. In fact, we found the panel for only $84.95 at Golden Star Classic Auto Parts. Of course, this Pontiac project would go onto include several additional sheetmetal parts, but as we mentioned, we’re focused only on the floorboard in this story.

No matter which path you choose, DIY or a professional repair, the bottom line is you can’t ignore your project car’s corrosion indefinitely.

Rust, as they say, never sleeps.

Bare '64 LeMans convertible body on rotisserie
2. The starting point for the project is this comprehensively corroded ’64 Pontiac LeMans convertible, which suffers from the same rust issues as many vehicles of its era. The body was first media-blasted to reveal the extent of the corrosion. A layer of primer was sprayed on the bare sheetmetal after blasting to prevent further oxidation during.
Old sealer from previous floorpan repair
3. Not surprisingly, the stripped body shell revealed previous repairs, including this quick-and-dirty patch for the driver side floorpan. It was simply a sheet of steel tacked in place and surrounded by a hastily applied sealer. At least it kept the driver’s feet off the ground.
Old sealer from previous floorpan repair on passenger side
4. It was the same story on the passenger side front floorpan. Worse even. Fortunately, the transmission tunnel was comparatively solid, so the metalwork would involve replacing the individual floorpans rather than the entire floor of the car. The trunk, however, was another story and it would require a complete floor replacement.
Rust on bottom of floorpan
5. Here’s a bottom-up look at the rot and previous repair. It ain’t pretty, but fortunately the damage is confined to the floorpan itself. Even better, the crossmembers and other structural components are intact and rust-free.
Air hammer used to cut into old patch panel
6. The first step in the repair is the removal of the previous patch panel. It doesn’t have to be pretty at this point, so an air hammer was used to simply cut around the inner perimeter of the panel.
Extensive rust on body underneath removed patch panel
7. With the old patch panel removed, even more corrosion is revealed in the layer beneath, the remnant of the actual floorpan. It all must go. Fortunately, the body-on-frame design of the A-body means there are fewer crossmembers and spot welds to contend with in the floor compared to a vehicle with unitized chassis.
Plasma cutter used to removed large section of corroded floorpan
8. Several methods could be used to slice out the corroded floorpan, from a torch to a cut-off wheel to even a pair of shear if you really wanted to make a day of it. A plasma cutter is used here, and it works by sending an electric arc through compressed air or another gas that’s passing through a constricted opening. That elevates the temperature of the gas dramatically, up to about 20,000 degrees F. An oxyacetylene torch reaches about 6,000 degrees F, meaning the plasma cutter flows through the sheetmetal like a hot blade through ice cream.
New replacement floor pan from Golden Star Classic Auto Parts
9. The replacement floorpan for the repair is available from several restoration parts suppliers, including Golden Star Classic Auto Parts. It is contoured to match the car’s original stamping and is designed to be trimmed to fit.
Tracing new floorpan on body to mark trim lines
10. The new floorpan was laid over the opening of the original floor and the edge traced to indicate the trim line for the rear of the pan. The sides of the pan will be mostly cut off because the portion that extends over the transmission tunnel isn’t needed. It would be easy, and lazy, to simply leave the pan as-is, but taking the time for a custom fit will pay off with a more precise fit.
Cut-off wheel used to cut into body
11. A cut-off wheel is used to simply cut out the additional metal in the floor to make room for the new floorpan. A clean, sharp disc makes the job easier. A dull one will require more time to generate more heat, which could possibly warp the metal.
Grinding disc used to reach baremetal around opening
12. After that, a grinding disc is used to dress the sheetmetal and prepare it for welding.
New floorpan test fit
13. The first test-fit of the floorpan looks “OK” at a glance, but upon closer inspection, it’s easy to see the mile-wide gap between the pan and the toeboard structure (upper-left corner) and the central reinforcing “beam” of the pan doesn’t align with the corresponding area at the front of the body structure.
Gaps between floorpan and body near firewall
14. Here’s a closer look at the alignment. More trimming of the new floorpan and probably a little hammer work is in order.
Further trimming of floorpan
15. The edges of the floorpan are trimmed further. The goal is to get the pan to fit into the opening, sort of like a puzzle piece. It doesn’t have to be exactly that precise, but the idea is for it to butt up against the edges of the opening with no overlap.
Gaps tightened up after more trimming
16. Getting closer. A couple of sheetmetal screws snug the new pan into the opening, drawing it down further for a more precise fit.
Tracing new trim lines on floorpan for better fitment
17. The tighter-trimmed pan’s bottom edge is traced for further trimming. Again, the idea is to create a new panel that fits into the opening without overlap.
More trimming done with plasma cutter
18. A quick zip with the plasma cutter trims the panel’s edges down to size.
Tight gaps between body and floorpan after trimming
19. The pan is back in for another trial fit and shows it’s about as close as it’s going to get. The alignment of the center reinforcement is good, although it’s clear to see the widths of the factory stamping and the replacement stamping differ. That can be further massaged for a totally seamless appearance.
Floorpan clamped in place for welding
20. Here’s a look at the custom-fit floorpan, clamped in place and ready for welding. The attention to detail that came from Wing’s continued trimming is terrific. Note how the horizontal ribs in the trans tunnel align perfectly with the ribs in the new pan. Nice work. Also notice the floor drain hole is traced for cutout.
Initial small welds begin
21. The first welds are applied as small, individual stitches all around the perimeter. Leaving a couple of the sheetmetal screws in at first helps ensure the panel stays put and that the car’s panel and the replacement pane remain aligned for perfect butt-welding.
Further welding
22. Wing alternates the weld stitches around the panel until the entire perimeter is filled. Alternating the welds prevents warping from too much heat building up in one area. He’s using a MIG welder, which is preferred for sheetmetal like this.
Floorpan fully welded in place
23. Here’s the fully welded replacement floorpan installed, including the drain hole cutout and massaged to provide the radius appearance that mimics the original stamping process. Typically, the drain plugs are sold separately.
Underside of new floorpan after welding
24. From the bottom, the new pan looks great, and it is as strong as new. Structurally, the car doesn’t need further welding or metalwork, but depending on the level of restoration, further finishing work could be done around the edges.
Corrosion and sealer on old patch panels on rear fender edge
25. With the driver side floorpan installed, the passenger side pan needs to be replaced and then it’s onto the rest of the car. It’s a seriously perforated Pontiac that’s being restored one replacement panel at a time.
Miller Big Blue 800 Duo Pro SF and FieldPro Smart Feeder
Welding 101
Welding is as much an artform as it is a skill; no matter how you approach, practice and experience are the only ways to become proficient at it. Not surprisingly, many enthusiasts are reluctant to make the investment in a welder for fear that their trepidation in learning the skill will relegate the unused machine to the corner of the garage, like a 220-amp treadmill.

There isn’t a single welding process to cover every task, but certain types are more likely to be encountered in the types of projects associated with working on cars. Here’s a quick rundown:

Stick Welding: It’s the most basic of welding types and is typically what many people use as their first foray into welding. Also known as arc welding, it simply used an electric current flowing from a gap between the metal and the welding stick. It’s good for basic work, like welding hitch receiver to the bumper frame of your truck but tends to throw off a lot of spatter and can’t be used with anything thinner than 18-gauge sheetmetal.

MIG Welding: MIG stands for metal inert gas and the process involves a wire-welding electrode on a spool. An arc created by the electrical current between the metal and the wire melts the wire to join the metals. It’s a relatively easy process to learn, although it takes experience to learn optimal temperatures and stitching procedures. It’s also a clean process, with little or no spatter, and it can be used on thin or thick metals.

TIG Welding: TIG stands for tungsten inert gas and is an arc welding process that uses a tungsten electrode to produce the weld. The tungsten does not create the filler, like the wire in MIG welding and with some processes, one isn’t needed. TIG welding is generally the best for joining thin sections of metal and non-ferrous materials, such as aluminum and magnesium. It is more complex than MIG welding and more difficult to master.

Bottom line: A MIG welder will handle just about all the automotive hobby projects you’ll encounter. It’s the just-right tool between a basic stick welder and the more-complex TIG welder.

New Door Striker Shims for Mopars
Red '69 Charger R/T
When it comes to reversing the rust on a muscle car–era Mopar, most enthusiasts are stuck with the often-corroded original door striker shims or take-offs from a parts car—and that’s only if better ones can be found.

It’s a compromise that can be avoided with JF Restoration Parts’ new laser-cut shims. Like the factory shims, they install behind the striker on the door post to optimize the door latch catch and door alignment. The shims come in a kit with four sizes that match the thickness of the originals: 0.032, 0.048, 0.060, and 0.077 inch and each kit includes two of each, which is enough for a two-door car.

The shim kit is available on eBay, under item number 222204828682, or by searching “jfshims;” or call (204) 822-0745 for more information.

Golden Star Classic Auto Parts
(972) 315-3758
goldenstarauto.com
Modern Rodding
VOLUME 5 • ISSUE 41 • 2024