GTO Floorpan and Trunk Floor Replacement

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in GTO, GTO Restoration

Floorpan and Trunk Floor Replacement
Floor and trunk pans are areas that are frequently rusted out from continued exposure to moisture and salt. Depending on where your car is from, the damage could be as minor as pinholes in a foot well to entire sections no longer being present. Fortunately, the solutions are as large or small as the damaged areas.

MIG-welding body panels—whether it’s fenders, quarter panels, or floorpans—is an art. If you have not used a welder before, it’s wise to take a class and develop the necessary skills. If you’re a quick learner you can practice in your garage. Body panels are typically 14- or 16-gauge sheet-metal steel, and if your MIG welder is set too high or you apply too much heat to a particular area, you can easily burn through the sheet metal and ruin the component.
Therefore, set the welder at a lower voltage for welding thin metals. I recommend at least practicing the tack and stitch welding required for the floor installation on some scrap metal before attempting to install the floorpans. The patch panel and the existing sheet metal should overlap slightly and the gap should be minimal so fill welding is not required later. Apply the weld every 4 to 5 inches, quench the area with water or air, and then apply the next tack weld. If you don’t allow the welded area to properly cool before applying the next tack weld in the same area, the panel warps. If it warps, corrective measures must be taken using a hammer and dolly to straighten the body panel, which is time consuming and frustrating. Tack weld the patch into place and inspect the repair for fit. When it is deemed satisfactory, completely weld in the panel, making sure that the heat is not so hot that it warps the metal. Then grind the weld flush and prime it. (Photo Courtesy Scott Tiemann)

Before reproduction floorpans became available, repair methods ran the gamut from using NOS sheet metal obtained from dealers to cutting floors out of rust-free Western cars to welding in fabricated patch panels. Other more creative methods, such as using expanded sheet metal covered with fiberglass cloth has been used, as was riveting on a piece of sheet metal or even just laying a piece of sheet metal over the hole and letting the carpeting hold it in place.

For some reason, where I come from in Upstate New York, it was all the rage in the 1970s and 1980s to steal stop signs and cut them up to make floorboards. I’m not sure what the reasoning was, as the metal used was much thicker than production car floorboards and was not even steel—they’re silk-screened aluminum. I suppose it was more the thrill of obtaining the stop sign than its suitability as a replacement panel.

In any event, the aftermarket came to the rescue, saving these would-be scofflaws from months of possible jail time and community service, as well as helping law-abiding restorers regain the structural integrity they lost to the elements.

Replacement floorpans and trunk floors are engineered to fit right and restore integrity to the body. Like the originals that they replace, the reproduction floor panels are stamped from the correct-gauge sheet metal and feature the same ribs and contours as the originals to make their installation as trouble-free as possible. Companies that manufacture and sell these panels include Year One, Ames Performance Engineering, Original Parts Group, Performance Years, National Parts Depot, and Dynacorn.

Further aiding the restorer are the sizes of replacement pans available. In some cases, the rust damage is limited to one area, such as below the accelerator, brake, and clutch pedals.

The factory carpeting has a vinyl mat area that can trap moisture from wet footwear, leaving the area prone to rust-through. For jobs such as this, quarter sections are available for front and rear on both sides.

If the support brace just behind the outer sheet metal is also rust damaged, cut and weld in a patch as on the outer-section repair. Grind the repair smooth so the fender regains its original appearance and integrity. (Photo Courtesy Scott Tiemann)

Once the floorpans are rusted and weakened, they need to be replaced. Several tools can be used to cut out the rusted-out floors of a GTO. Most use a plasma cutter. If you use a plasma cutter, use it with care and precision. You can inadvertently cut through the frame rail, floor support, and other components much faster than you’d think. So be deliberate and careful because you don’t want to patch and repair those from inadvertently cutting them with a torch. To remove the floors, use a rotary cutoff tool, which takes some time and is hard work. Use a 3-inch disc in 1/32- or 1/16-inch thickness, and do yourself a favor by buying a high-quality disc, such as one from 3M. Use the cutoff wheel to precisely cut out the rusted and weakened factory floorpans. Use the replacement pans as templates for cutting. Take precautions to ensure the original floor bracing is not harmed in the process. Though these look bad, they are actually in great shape. Most of the rust seen on them is from pieces falling from the floor panels, not from the braces. (Photo Courtesy Scott Tiemann)

The floorpans are a major structural component of the body, and therefore when cutting out the floors, the body can move out of alignment. You don’t want to install the floorpans when the body isn’t square. Hence, you need to install these brace bars and take the proper measurements to ensure that the body is square. This is not really necessary for a floorpan replacement, but if your car is also getting quarter panels it needs the support. These floor braces have been soda blasted and primed. Once the floor braces are stripped, the pans are ready for installation. (Photo Courtesy Scott Tiemann)

While some purists may use a clean floor from a donor car, you can install a reproduction floor on your GTO. Mark the alignment for the new floorpans, then measure and cut for a butt weld to the existing sheet metal. To maintain accurate alignment throughout this tack welding stage, drill holes around the perimeter of the replacement sheet-metal floor to hold it in place. This is done to locate them and keep them in the proper position during the welding process. Also drill holes in the areas just above the floorpan supports, so they can be easily welded to it. Then, using a MIG welder, place tack welds every 3 or 4 inches along the floor braces. Be careful when applying these welds because you don’t want to burn through the thin sheet metal or apply too much heat, causing the floors to warp. (Photo Courtesy Scott Tiemann)

Replacement floorpans also come in left and right halves, front to rear, as well as full-floorpan stampings. These replace the entire floor from the rockers to the firewall to the wheel wells and trunk. They can even include support braces and rocker panels, if you wish—essentially the entire bottom of the body. This is an attractive alternative in cases where the rust is severe, though some restorers prefer to weld the smaller pieces together, as they are easier to handle than one large piece and save money on shipping costs.
Keep the condition of the passenger compartment floor and the trunk floor in mind when evaluating the car. Make sure that you are poking and prodding the entire area with a rubber mallet and a screwdriver. In addition to rust holes, look for weakening of those areas with heavy surface rust.
If you find that you can get the floorpans to deflect even a little with your hands, it is a clear indicator that their strength has been compromised by corrosion. They should be replaced, even if there aren’t any visible holes because the structural integrity of the body is compromised, and ultimately it’s a safety issue. You don’t want to have seats falling through the floors six months after the car is finished. Take the time to carefully evaluate what you have while the car is all apart, because it will never be easier to weld in new floorpans than it is at this stage.

Professionally installed floorpans look similar to this when viewed from underneath the car. The seams, panel stamping, and alignment with the floor braces make it appear as the original one-piece unit. Grind down the tack welds and prime and paint the floor. Once the area is properly finished, the repair is nearly invisible and restores the integrity of the body. A floor replacement, such as this, adds value to the restored car. On the other hand, if the stamping and panel alignment is off, a potential buyer or show judge will recognize this issue and the car will be worth less and points deducted. Replacing floors for the first time takes care, attention to detail, and diligence. (Photo Courtesy Scott Tiemann)

The procedure for welding in new floorpans involves careful measuring and cutting away of the original floor. The idea is to remove only as much metal as necessary to properly install the panel to solid, original metal. Be careful to avoid cutting the support braces; they may be reusable. If you are finding that you are cutting into more and more rusted metal, keep cutting until you find solid metal, even if it means having to purchase additional panels. What good is it to weld in fresh reproduction sheet metal to thin, corroded, compromised sheet metal? Again, safety and structural integrity is what you are going for.

A thorough inspection of this 1970 GTO Judge convertible revealed major rust damage in several areas. In particular, the quarter panels suffered major rust damage, and these parts required replacement in some cases and major repair in other cases. Water spray from passing cars often leads to more rust on the driver’s side of the car, and in this case, the driver’s side is far worse than the passenger’s side.
Although the driver’s side was worse than the passenger’s side, the passenger-side quarter panel still needs to be replaced. Use an air-powered rotary cut off tool to cut out the rusted outer skin below the beltline. Use the replacement panel to guide the cut off tool across the rusted skin and achieve a straight cut. Drill out the factory quarter-panel spot welds, so the panel can be removed. Use a special spot weld drill bit, which drills a 1/8-inch hole in the center and then cuts away the top metal in the spot weld until it is freed from the bottom half of the weld. Note that this rocker panel is still in very good condition and the outer wheel well only has minor rust-through along the edges. (Photo Courtesy Scott Tiemann)

Once the corroded metal has been removed, the area should be prepared with a grinder or reciprocating saw to smooth the edges. As with the patch panel described earlier, the new pan should be tack welded every 1 inch or so and checked for warpage and fit. Once that is achieved and the job is satisfactory, finish welding in the pans and then grind down the welds for a correct appearance.

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