New 1969 Mustang Project: Floor Pan ReplacementBookmark this
It seems as though you never have the time to get to the projects that are most important. Seven years ago, we picked up this 1969 Mustang Coupe for my son Jason to build as a father and son project. He was 12 at the time, and the car needed a fair amount of work, but we figured we would slowly work on it. Now that he is 17 and about to enter his senior year in high school, it runs and can move under its own power, but it is not streetable—not even a little. Over the next few months we will be attacking this Mustang pretty hard to get it safe and legal for Jason to drive, something we’re sure a lot of readers and their families will sympathize with.
The Mustang is a full project. There is no interior, it’s got rough body work, and as you are about to see, it has no floors. The straight-six under the hood was rebuilt and runs pretty good now. We recently went through the front suspension, so all of that is new. Now it is time to get into the meat of this coupe and make it driveable. That means replacing the driver-side floor pans. Once the floors are solid, we can move on to making this shell of a Mustang an actual driving vehicle, with a new interior, paint and body work, rear suspension upgrades, and eventually a V8 conversion. The good news is that even though the coupe came with a straight-six, it had all the other features you want, like power brakes, power steering, and even A/C.
At some point, the previous owner had patched the floor with a piece of tin over a giant rust hole. There was essentially nothing left of the original floor beneath it. The subframe was free-floating, it was that bad. Fortunately, the damage was concentrated in the center of the footwell, so a single floor pan patch panel would work, though there were a few small areas that we would have to fabricate some pieces for. This patch would also require replacing the original seat stand, which is a separate piece of formed sheet metal. These components were ordered from Summit Racing.
If you are not the world’s best welder but can make two pieces of metal stick together, then you have enough skill to weld in a floor pan. It is actually about the best practice you can get, as it doesn’t matter what the welds look like, as long as they are solid. Most of the welds in this project were done by Jason, who has welded before, but not often. You will need a few tools to perform this project, including a welder (flux core or gas-shielded MIG preferred), grinder, cut-off wheel, spot-weld cutter or air-hammer with spot-weld breaker, small body hammer, drill and bits, clamps, and if possible, a plasma torch. You don’t have to use a plasma cutter, but it really speeds up the process if you have access to one. Otherwise, you can use a cut off wheel.
Before anything gets cut, you must check the fit of your patch panel. You must fit the patch before cutting anything—this is critical to a proper repair. We are using the entire panel, so we simply placed the pan in the car, marked the perimeter with a marker, and made some alignment marks. The alignment can shift after the old floor is removed, so having the correct placement marked on both the pan and the original floor is important for a good fit. We used the plasma cutter to remove the old floor; leaving a quarter-inch lip around the perimeter where the new pan will rest. This is a lap joint, which is much easier than attempting a butt-weld seam, which is just not necessary for floor pan repairs.
Because Mustangs use a unibody chassis, the floor pan is a critical component. This means that you have to install the pan in the correct position and replace all of the original spot welds. The key areas for structural rigidity are the front subframe and crossmember braces under the forward footwell. If you are replacing the rear floor pans, the torque boxes must be welded to the floor pan as well. With the replacement pan in position, the factory subframe components were traced onto the new pan from underneath, and then we drilled 3/8-inch holes every couple of inches for spot welds. You want at least as many spot welds as the factory had—we used more to ensure that there were plenty of solid weld points.
Before actually welding anything, you need to prep all of the surfaces. This means removing any rust or scale, grinding to clean metal, then prepping the clean surface with a high-zinc weld-through coating. This special primer protects the fresh welds with a coating of zinc when the metal is welded. Without this, the fresh welds are subject to immediate rusting, which you don’t want. After the welding is completed, we treated all of the seams to a coating of seam sealer.
Replacing a single-side floor pan is about a one or two day job, depending on the level of experience and rust that you have to fight through. We made a few patch panels with some scrap 16-gauge sheet metal where the floor pan was rusted but the new pan did not cover. Floor pan replacement feels like a big job, but it really isn’t. In fact, it is quite forgiving of mistakes. Take your time, be patient, triple check your placement, and weld ’er up!
|floor pan||GMK-3020-505-64L||Summit Racing||$30.99|
|weld-thru coating||generic||local store||$19.99|
|seam sealer||generic||local store||$9.99|