Most Mustangs suffer from floor pan rot. In some degree, our cars all suffer this. Like most of us, we want to do something about it, but neither have the time, tools or we DON’T think we have the ABILITY.
I can do many things, but floor pans seemed a challenge, and some of the articles I researched increased my image of the difficulties. However, after getting an estimate from a couple of body shops and local resto or custom shops it didn’t add up. The going rate was around $1500.00 bucks! Per side.
With this shocker, I checked the cost of pans in the catalogs; long fronts are $29.00 bucks or so. I also did a lot of research on welders (Ford muscle – big help from you folks), came up with a ball park of $500-600 for a decent welder (Lowes came in at $449 using the 10% military)! That certainly covered the cost and made sense, now for the time and know how pieces. Time I own, its mine to manage and spend as I want. The know how, again turned to Ford Muscle and other sources.
Here is where it got convoluted. The advice ranged from “pop rivet some aluminum plates in there to cover it up” to “full on shop restoration type surgery”. However, the articles, aside from making it look like you need to be a pro, seemed to leave out a lot of info that I suspected I needed.
So I dove in and found out its not rocket science. It does take time, but it certainly is worth doing! Its fun rewarding and I can say I DID IT, I didn’t stroke a check – I DID IT, most of all I saved a lot of money and developed skills and knowledge that will carry over to other projects
Entering arguments: depending on the condition of your car it may vary, as to how much a floor job you want. Mine was solid, it only needed front longs, and I already had the sub frame connectors installed, so I didn’t have to worry about sagging or splaying. Also the condition of your sub frames, decide if you need too, or want to tackle those as well
Knowing what I know now, I would go ahead and put the complete front and rear pan in. The front pans are the challenge, doing the front and rear pan would be a piece of cake, not to be confused with the whole floor assembly, different animal.
Now to boil this down the average home build garage level, you will need at least a 25 Gal, 2 HP compressor and cutoff wheel (I actually blew up my father in laws Craftsman, and had to purchase a 60 Gal 3HP). A MIG Welder, I bought the Lincoln 140 but this job could have been done with the Lincoln Handy Pack (but get the gas). Finally, spot weld cutters (I went through two cutters and about 9 heads on this project). If you can’t weld at all, take a course or watch the directions. I already knew how to stick weld, it has been 30 years and some of that being underwater – the MIG took a bit of getting used too, but if you can run a glue gun, you can MIG. The other tools are basic shop tools, angle grinders, hammers, jack stands etc.
This is the ultimate project to start on, its really not that difficult, the welder and maybe compressor are items you will need and use in your project, further reducing the overall cost of your Mustang project. The cost of the tools, pans etc are still cheaper than paying someone. Also, if it does not turn out pretty, you will learn, and it will all be covered by carpet etc.
The floor pans I used came from Mustang Unlimited, all the companies are about the same. Of note, all floor pans for 64 1/2 to 70 Mustangs have the same lip on them. Meaning they are stamped for a convertible. The rocker lip on the convertible turns down where the coupe and fastback turn up. I didn’t want to spend a lot of time under the car putting in rosette welds (plug welds) so I took them to a sheet metal shop, and for $10.00 bucks they reversed the brake on the lip. This allowed me to do 95% of the work inside of the car.
Jack the car up, support it with six stands, two under the rear sub frames just in front of the leaf spring eyes, two under the front sub frames, and two under the front frames where it meets the radiator support. Your car should be square, now is a good time to check it, I did and it was, also the car should be level as you can get it. Hint, if she is resting on all four jack stands, and its level – guess what, its probably pretty square, certainly enough for floor pans.
Once up on stands, take the entire interior out of the car, except the headliner and dash. Then scrape all the sealer and goop and gunk off the floor seams and seat bases. You will then remove the seat bases. Take a hard look at the bases, even if it doesn’t look rusted underneath, odds are they are. The edges of the bases will look ok, and the under side of the car will look ok, but more than likely you have a lot of rust between the base and the floor. Go ahead buy a couple; they are fairly inexpensive, for piece of mind, replace if in doubt.
Before removing the bases, take some measurements, you can’t measure and mark enough. I pulled measurements off the back of the seat bases to the rear floor pan, I ensured the upper seat holes were projected over the rocker panels using string, rulers and sharpie pens, noting the front and back of each seat hole, to ensure I got the bases back in where they came from.
Pulling the seat bases is where you experience the first chance at the spot welds. Depending on how rusty they are, you may not see them. If you can, your bases may be ok. If you can’t see the welds, the easy way to find them is slipping a thin scrapper between the base and the floor until you can work it in, and pry up bending the lip a bit, which will outline your spot weld. Drill a small hole in the center, run your spot weld cutter through it, don’t worry about punching through this, as you are replacing the floor pans. I had to run down one by one to find my spots on the seat base, vice going along and drilling them all, then cutting.
The front and rear of the seat bases are spot welded, the sides along the rocker and transmission tunnel are stitch welded and will need to be ground off with the 4 inch angle grinder. Once all the cutting and grinding are done, remove the seat base, and there is your floor. Even if your replacing your bases, keep the old ones, I found it great practice running some welds on the old ratty bases. If you can weld on those, new metal is a sync.
With your bases out, re asses your floor pans, and be sure your replacing the right amount, Its better to replace metal in question than trying to weld on rusty metal. Also, pay close attention to where your bases ended in the rear, on a long front panel this is right about where your weld seam will be, and if the area between the pan and base are rusted, this is a bear to weld the new pan back too.
Now, with your new pans, it is time to set the alignment and see how much metal you have to work with. Lay the new pan over the old one, line it up with the floor plugs and seat bolt holes. I marked the front and rear bolt holes on each pan with an A and B mark that I transcribed on the rocker where I have previously marked the outer and center lines of the seat attachment bolts. Great reference for ensuring it all goes back the same way. Also run a sharpie along the floor pan flange along the rocker to establish your replacement height alignment.
Now with the seat pan laying on top of the old floor, have someone push it down as far as you can to the floor, and take a white china marker (get them at an art store – easy to see but wont wipe off too easy like soap stone), and mark the outline or edge of the floor pan onto the transmission tunnel, and rear floor pan.
Remove the new pan; the marked outline will give you a guide as to how much metal you can remove up the tunnel side and establishes your rear cut along the back of the old floor pans. The fronts also come with extended toe board pieces, if you need you can also cut up into the toe boards.
The point of the outline is so you won’t remove more floor than you have new pan material. Again, if using long fronts pay close attention to where the seat base lands on the floor and where your cutting, adjust your rear cut to ensure you have enough new pan, and try to avoid having welding your base on top of your rear pan seam weld.
Now you’re ready to start removing the old pan. It’s important to not cut your frame rails or sub frame connectors – duh! No I didn’t do this, but its worth noting if you get to sporty. So how do you know where to cut from inside the car? I went under the car with a drill and a small bit, probably ¼ or so, and drilled or perforated the old floor pans where the frame rails and transmission cross members were. I then marked the perforated lines with china marker, and went ahead and made these cuts to separate the floor from the frames and cross members.
This allowed me to cut out large sections of old pans, then work the old metal off the top of the frames. I tried to use a reciprocating saw to make the big cuts, but it does not cut straight enough lines for mating edges and butt welds. The straighter the cut, the better the fit and weld. Removing big chunks of floor pan allowed me to get in the car, and put my feet on the floor to work comfortably.
Cutting out the transmission tunnel side requires a good straight edge. I don’t have a plasma cutter, very expensive, so the straightest cut I could make was using a cutoff wheel and marking and guiding the cut with blue painters tape.
This is where the perforations and marking around the frames and cross members came handy, cut slowly and carefully over the tops of these. I found it easiest to estimate the center of the holes, and measuring back from the drill holes to get a mindset on where the flanges ended. I then cut into the hollow part of the cross member and work my way toward the edge with the cutoff wheel. You can estimate where the lips are and you will feel it when cutoff wheel meets the cross member or frame edge, and not do any damage.
Along the rocker and leading or front edge of the floor pan, it’s more of those pesky spot welds, and I found the fastest way to find and cut them was starting at an edge, working the scrapper under it, lifting the metal to reveal the spot weld and then hitting it with the spot weld cutter, repetitious? Yes, this project doesn’t go fast. You may get lucky like me. A lot of my welds were rusted and a lot of them came out with a good gasket scraper and hammer. The scrapper and hammer are also really handy if you don’t quite catch it wall with the cutter, just tap it on out.
Now for the fun part, removing the metal from the frame rails. Same as before, sometimes you can sand or wire brush the old floor to find the welds, but that really didn’t work to well for mine. I just started by opening up an existing rust hole to see the frame rail insides, and then started working my scrapper under the old metal, lifting, drilling, and then cutting. It really went well, and didn’t take too long.
Congratulations, you should have the old floor pan out now. This works for both sides, absolutely no difference. Don’t scrimp though, if you’re in doubt about the metal, any pitting etc, cut it out. Rookie mistake for me was not cutting far enough, and having to weld in a strip to get the weld to hold to good metal.
Now asses your frame rails and rockers. If you have rotted frame rails, you will have to replace those. Different tech article there. If your rocker is rusted a lot, you will either have to replace that, or patch in new metal. There are articles on that as well, pretty easy though. I was lucky, the only thing I had was frame rails packed full of red southern clay. Surprisingly, there was no rust in there; the metal still had the galvanized finish on it.
Time to prep the metal for replacement. I cleaned and vacuumed out the frame rails, hit them really well with POR 15 metal prep, then POR 15 as much as I could inside the rails. I hit all the remaining edges with an 80 grit abrasive wheel to ensure all old paint and sealants were gone. Then I sprayed a thin coat of high zinc, weld through primer on the tops of the frame rails, cross members, toe boards and rockers.
Time to trim and fit the new floor board; this is the most tedious part of this project. Lay the new pan in over the cut hole, line up your bench marks on your rocker and pan. It won’t fall through; your pan should be way bigger than your hole. Now have a friend, wife, or use a couple of Optima batteries to put on the new pan, you want to push it down as far as you can into the floor ensuring its making good contact with the sub frames and cross members. Go under the car with your china marker, and mark a line along the new pan, where the transmission tunnel and rear floor pans meet the new floor pan. This is the area to be cut out.
Careful here, don’t cut it all at one time! As you cut and remove the pan edges, it will allow it to settle into the car a bit more, and if you cut it all at once, it may be too much, leaving excess gaps at the edges. I cut the excess off to within a ½ inch of the line, and then repeated the process again, and again, and again. This is where the china marker came in handy; I could erase the mark with a shop towelette (wet one for shops) and remark the new pan (FE said it best, measure, measure, measure). Also, as mentioned above, I found using blue painters tape to guide the cut a huge benefit. I also made these cuts with a cutoff wheel.
You don’t want to remove too much metal, the tighter the fit, the easer the weld. Once I got it within a ¼ inch of fitting, I then would use the blue tape as a guide/marker and I literally ground it down a 1/16 of an inch at a time to get it so it would pop into the hole and fit. Every time you mark and check it, ensure your pushing it down onto the cross members and sub frames. Don’t be afraid to tap it down with a rubber hammer, especially the bends at the transmission tunnel and trans cross member and toe board areas.
Now your floor pan fits, its time to mark the transmission tunnel and cross members for the plug welds. With the floor pan in and weighted or held down by a friend (I also used the drain plugs and seat holes to pull on to cinch them down), I marked a series of x’s where the transmission cross members made good contact (even from the factory they don’t and you can’t get 100% contact on this). I outlined the frame rails as well with my china marker.
Take the pan out of the car, and measure the flange on the frames and cross members, and measure half that distance from the marked edges of the frames to locate the frame flange center onto the floor pan. Drill a series of 5/16 holes 1 to 1 ¼ inch apart along the sub frame flanges, and do the same along the rocker lip, toe board lip and transmission cross member. Now your plug weld holes are set.
Finally, run an 80 grit wheel or wire brush over the contact areas of the toe board, rocker panel, and frame rails and cross members to remove all paint. I sprayed a thin coat of weld through primer on the areas under the car.
Now you’re ready to fit and weld. Ensure your frame rail flanges are tapped up and at least level if not above the frame rail. The easiest way to get this thing in place, and hold it while ensuring good contact for the welding is to line up your bench marks, then take a small drill bit and some small self tapping screws. Drill holes in the rocker, and frame rail flanges, and toe boards and screw that sucker down tight.
Do this wherever you don’t have good contact. Then take a ball peen hammer and tap down every plug weld to meet the rail, or metal your welding to. You should not see a gap in the two mating surfaces you’re welding, otherwise it will burn through.
Run some very small tacks on the rocker lip, corners and middle of the transmission tunnel, check your fitment. Then using rosette welds, weld the pan to the frame rails. I worked from the middle back then forward. I would weld a short series of stitch or tacks along the transmission tunnel, then a couple of corresponding rosette welds along the rocker, then stitch along the rear seam, then a couple of rosette welds on the toe board. Keep working back and forth, weld in very short bursts to minimize heating, blow through, warping or pulling.
You will have some seams that will not come flat or flush together, or the gaps open up, see my tech article on Floor pan seam puller/heat sink.
Once the floor pans are welded in, then its just a matter of grinding off the excess bead, welding the seat bases back in and putting on some primer, seam sealer and paint. Congratulations, you did it, you can be proud of it. Best part, if it’s not pretty, its covered by insulation and carpet! You’re done!