While the picture above right...
While the picture above right gives some idea of the condition of the passenger side front floor, this close-up really shows what we were dealing with. There's no option here but to replace the sheetmetal if you want to do the job correctly.
While it's only been a couple of months since we brought you the huge undertaking that is the floor replacement in Editor Fortier's '47 Chevy, we recently replaced a front floorpan in a friend's mid-'60s Impala. It occurred to us that such rust repair is far more common than replacing an entire floor, and the procedure would be of use to many readers attempting to perform such a repair for the first time. It's an easy "Saturday morning" job, if you know how.
Replacement panels are available these days for a huge number of older vehicles, but unless you're a practiced bodyman, it may not occur to you to only use a portion of the new panel, simply fitting the whole piece, and making a lot of unnecessary work for yourself. The front passenger side floor in our subject Impala had rusted through, but only in the middle. (Why only this side, when usually the driver side rusts first? Possibly a leaking heater in the past?). While the patch panel from CARS was a good fit, with all the factory pressings and swage lines faithfully reproduced, there was simply no reason to replace the side of the transmission tunnel or the lower part of the toeboard in this instance.
Given that there's a stock seam across the floor just below the latter, it made sense to trim the new panel to fit as per the original, as well as to trim it to be welded on the flat section of floor rather than get involved in cutting the tunnel. While it was trimmed outside the swage lines for ease of cleanup later, we were able to reduce the amount of welding required quite substantially, retain more of the original floor, and make a neat repair at the same time. Of course it also meant we didn't have to remove any of the Dynamat that the owner had already installed on the majority of the floor, but we'd have trimmed the CARS panel anyway.
Safety First!
Be extremely careful when replacing floor sections in any car, but especially one that is currently being used on the road. Check the location of fuel and brake lines, and move them as far away as possible from the repair area. It's very easy to weld to a fuel line and ignite the entire vehicle. Don't ask me how I know! Never rely on others to tell you where the lines are; check for yourself. Check the condition of the lines too, for leaks, in which case they should be replaced anyway, and cover them with fire-retardant blankets (or aluminum sheeting, as weld spatter won't stick to this) during cutting and welding.
Keep a fire extinguisher on hand too, as it's easy to ignite the oil and grease that is often present on the underside of the floor on older vehicles. We're not trying to scare you, just warn you of the dangers of this type of work!
We should also mention that the interior and inside of the windshield should be protected against weld spatter or sparks, again using a fire retardant blanket (available from Harbor Freight) or even cardboard taped to the glass.
Whenever any cutting or welding...
Whenever any cutting or welding is contemplated on a floorpan, the first thing to check is where the fuel line runs. On the Impala it ran along the inside of the passenger side chassis 'rail, very close to where we'd be cutting and welding.
Unhooked from the factory...
Unhooked from the factory clips, the fuel line was dropped a couple inches until it rested on the trans crossmember. Before we started cutting, it was inspected for leaks, then when deemed safe, wrapped in a fire retardant blanket normally used to protect paint, glass, or interior trim from weld spatter or grinder sparks.
With safety issues taken care...
With safety issues taken care of, work could commence. The replacement floorpan from CARS is a nice piece, and fits well. It appears to be designed as an overlay panel, whereby it's simply welded over the old floorpan, though the original could be removed to within an inch or so of the replacement's edge, leaving a lip to overlap and weld.
We prefer to make repairs...
We prefer to make repairs as invisible as possible, even on a floorpan. Besides, there was no need to replace such a large area and create unnecessary work by welding up the sides of the trans tunnel and toeboards when the rusted area was much smaller. Therefore, we marked the new panel to be trimmed.
Ignoring the obvious lack...
Ignoring the obvious lack of gloves or safety guard (this was for "photographic purposes" only, honest!), the CARS panel was cut to size using a 4-inch grinder and cut-off wheel from Harbor Freight.
With the trimmed panel laid...
With the trimmed panel laid in place, its perimeter was marked using a silver Sharpie, which would provide my cut line to remove the original floor section. Of note here is that the front edge of the trimmed panel is actually on a factory lap joint, so this will appear stock once we're done. Cutting across any swage lines was purposely avoided for ease of cleanup after welding.
An air saw was used to trim...
An air saw was used to trim the old floorpan away, partly because it produces no sparks (given that this is a regularly used car with oil and grease on the underside, and the proximity of the fuel line) and partly because it's often quicker than using a cut-off wheel.
We removed the old floor section...
We removed the old floor section by cutting an inch or so behind the lap joint at the base of the toeboard, then used the spot-weld drill to remove the welds. A chisel made light work of any remaining small welded sections. Note how the underlying sheetmetal is not damaged by the drill, and the fuel line is wrapped in a fire-retardant blanket.
With the old rusted floor...
With the old rusted floor section removed, the old paint was removed from where we'd be welding using a 36-grit pad on an air sander. The same tool was used to trim the edges to allow the replacement panel to fit perfectly.
At the rear righthand corner...
At the rear righthand corner of the section we're replacing there was a small brace under the floor, attached by a couple of spot-welds. Using a spot-weld drill, these were removed to free the floorpan.
A dedicated spot-weld removal...
A dedicated spot-weld removal drill has a small point at its center, but is otherwise flat across the bottom. The point allows it to make its own center while drilling out a spot weld without damaging the panel underneath.
Our old, trusty, manually...
Our old, trusty, manually operated joggler was used to put a step in the new panel along what would be its rear-most edge. This was the only edge that wasn't butt- or plug-welded, and was only done this way to illustrate another method of joining panels. Normally we'd butt-weld the three sides that weren't plug welded. The step at the edge was welded over the old floorpan's edge, allowing the new sheetmetal to sit flush with the existing floor.
While most everyone knows...
While most everyone knows to disconnect the battery when welding, here's a useful tip. If there's nowhere easily accessible to clamp the welder's ground cable to, it can be attached to a magnet as shown. Note the stepped joint in the foreground.
The same joint seen from below...
The same joint seen from below illustrates how it'll look factory once the welds are dressed and the floorpan is undercoated.
With most of the welds ground...
With most of the welds ground back, the floor is strong, watertight, and ready for another 40-plus years of service.
While the drill shown here...
While the drill shown here is from Snap-on, and the cut-off wheel is a Clarke item, all the air tools to do this job are obtainable from Harbor Freight. Air-powered tools are smaller than their electric counterparts, and make jobs such as this so much easier. Of course you'll need a suitably sized compressor to run them too.
Here's the underfloor brace...
Here's the underfloor brace we referred to a while back, now plug-welded back in position. The lapped joint at the rear of the new panel can be seen here too.
Looking from underneath, the...
Looking from underneath, the factory swage lines and stampings are as they should be, and an application of undercoating will see the repair as good as new.
Here you can see how well...
Here you can see how well the new sheetmetal fits, butted against the original in the doorstep. It was tack-welded at first, before seam welding. Making the joint here on the curved section made grinding it flush easier later.
Here's the plug-welded lap...
Here's the plug-welded lap joint at the base of the toeboard. The spot-weld drill was used to perforate the new panel prior to fitment.