floor pan install, cutting & fitment questions - Vintage Mustang Forums
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Old 06-24-2019, 04:31 PM Thread Starter
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floor pan install, cutting & fitment questions

Working on a 66 coupe. As in previous posts, no welding experience or sheet metal experience. About to become an expert.

I've watched a few youtube videos on removing, cutting and fitting new floor pans. I like how this guy in the video below does it..

Question 1
If the old front and rear floor pans are cut out of the car with some length of the old sheet metal remaining along the transmission tunnel, and toe board area, could you simply lay the new floor pan into place, then using a cutting wheel or plasma cutter, cut through the new and old sheet metal simultaneously thereby getting a finished cut that is almost a perfect match and is only a cutting wheel thickness in gap? Or is it too much to hope that the doubled up sheet metal will be able to be cut at the same time?

Question 2
In the video below at the 8:27 mark, it looks like the new floor pan edge is up against the edge of the transmission tunnel sheet metal. Is that true? Then it would be a butt weld. However, at 8:55 he says "perimeter weld overlap" Was that a mis-statement?


Last edited by ddaytona1; 06-24-2019 at 04:35 PM.
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Old 06-24-2019, 10:18 PM
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If I were to do this again I would use the full floor pan. Not as much welding and a cleaner install. Just my two cents.
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Old 06-24-2019, 11:50 PM
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I did both rear pans and a partial front pan. I overlapped and welded the interior 100% then seam sealed underneath. Not the most elegant solution but do all those butt welds and getting it all lined up perfectly seemed silly other than maybe a pride thing.

Hindsight I may have been time ahead by doing a full floor or a larger single piece but I had no worries of unibody twisting doing it in small sections

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Old 06-25-2019, 08:09 AM
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I have patched and doe the full floor pan. Definitely prefer the full pan. When patching, I choose to overlap. This is how Ford did it. Itís faster, easier and if properly sealed it will last a long time. I found the patch panels never match up to the existing pan correctly. Trying to get a perfect but weld is near impossible without a tremendous amount of work. Your idea of overlapping then cutting to be a clean seem sounds great, but I couldnít get it to work well. The replacement pan doesnít fit down tight against the original floor, so getting it lined up right is tough.
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Old 06-25-2019, 08:39 AM
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I overlaped, then peened edges and welded from both sides. As has been mentioned, if you have a lot to do, it might be better just to use complete floor pan. If I had it to do over again, that is the route I would have taken. At the time though, I was just starting and was too intimidated to tackle the whole thing.

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Old 06-25-2019, 09:58 AM
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unless you're going to grind the buttwelds down completely and paint the underside for a glass smooth appearance, there's no reason to do it. Overlap the panels and weld them solid, seam seal, and paint or undercoat. It'll be stronger lapped anyway.

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Old 06-25-2019, 10:13 AM
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I do it in 3 steps:

Step 1: I lay down the new pan on the old pan and use self tapping screws in a few places to screw the 2 together...I then trace the outline along the trans tunnel side and where the corner comes around the back radius(in other words, I am tracing the areas that get butt welded)

Step 2: I take the pan back out, cut about an inch lower than the line I traced...I also drill out all the spot welds along the inner rocker, the frame rail, and the rear ledge...removing ALL the old metal except the inch I have left from my tracing lineyou can see the original trace line in this picture...easy enough to see on a light color panel...just used a carpenter's pencil...on a black panel a silver sharpie or a soapstone is better...or even a score line using a nail.



Step 3: I then lay the new pan back in and retrace my original line...this line usually moves about 1/4" lower than it was before. I take the pan out and cut the newly traced line...this is what I end up with:



As you can see, the fit is so tight you can actually TIG weld it without using a filler rod or filler wire(not that I am using a TIG welder in these pictures...but that is only because I am not much good contorting myself around in a car while using a foot pedal...but it could be done)

P.S. I only butt weld the areas where its patched into an existing panel(IE, I butt weld the trans tunnel side because I am "patching" a full floor panel...I spot/plug weld everywhere the factory did)

P.S.S. And yes, I end up with a good dozen extra holes to fill...some like those near the seam line I drill to be able to get a hammer claw on a self tapping screw to be able to pull on it for panel alignment so its flush. Some of the self tapping screws like the ones at the frame rail are to make sure I pull the rail lip tight to the pan for plug welding

Last edited by wicked93gs; 06-25-2019 at 10:23 AM.
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Old 06-25-2019, 11:13 AM
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^^^^ This is what I did on mine after the fact that is why I would use a full pan if I were to do it again.. either way just do it correct and take your time.. the end result is what you are after... something someone else as well as yourself will appreciate.. not done just to say its done.
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Old 06-25-2019, 11:44 AM
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Quote:
Originally Posted by 2+2=GT350 View Post
^^^^ This is what I did on mine after the fact that is why I would use a full pan if I were to do it again.. either way just do it correct and take your time.. the end result is what you are after... something someone else as well as yourself will appreciate.. not done just to say its done.
I agree. I did not appreciate having to undo the previous owner's overlay job(He just welded a patch down on top of the existing floor without bothering to cut out all the rust even...it was pretty bad) and re-do it correctly, it just made extra work.
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Old 06-25-2019, 01:29 PM
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Quote:
Originally Posted by wicked93gs View Post
I agree. I did not appreciate having to undo the previous owner's overlay job(He just welded a patch down on top of the existing floor without bothering to cut out all the rust even...it was pretty bad) and re-do it correctly, it just made extra work.
Hey, I think that guy did my fenders too. Was there a quarter inch of Bondo to cover it all up?

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Old 06-25-2019, 04:02 PM
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Hey, I think that guy did my fenders too. Was there a quarter inch of Bondo to cover it all up?

Nope...that would show a degree of caring the PO of my car didn't have...he evidently didnt care that the floor looked like the night sky with a thousand pinholes in the metal around the patch. It baffles me as to why he even bought the correct patch panel(or part of it) instead of just cutting up an old fridge or something. But hey, at least his passengers feet no long went through the floor right?
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Old 06-25-2019, 04:56 PM
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When you're cutting out the floors make sure you know where anything important is underneath so you don't cut through the gas line or anything like that.
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Old 06-25-2019, 05:00 PM
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Quote:
Originally Posted by wicked93gs View Post
I agree. I did not appreciate having to undo the previous owner's overlay job(He just welded a patch down on top of the existing floor without bothering to cut out all the rust even...it was pretty bad) and re-do it correctly, it just made extra work.
Hey, this sounds like mine too, there was just some sheet metal welded in right over top of the old rusted out floor. As a bonus my PO went nuts with the seam sealer and now I have lots of tar to remove before I can even start looking at the rest. I think mine needs everything cut out from the firewall back to the rear seat.
By a full pan, are you guys talking about an entire new floor, or just the long pans for each side? My trans tunnel is really solid, but the floors have to go, specifically the toe boards.

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Old 06-25-2019, 05:17 PM
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Mine were incredibly poorly patched, so I bought the full pan. No major cutting, or excessive fitting with the patches or short pans. Less welding too, I can just plug weld it in, seal it up, spray it and carry on!
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Old 06-25-2019, 06:55 PM
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Originally Posted by Mr.E View Post
Hey, this sounds like mine too, there was just some sheet metal welded in right over top of the old rusted out floor. As a bonus my PO went nuts with the seam sealer and now I have lots of tar to remove before I can even start looking at the rest. I think mine needs everything cut out from the firewall back to the rear seat.
By a full pan, are you guys talking about an entire new floor, or just the long pans for each side? My trans tunnel is really solid, but the floors have to go, specifically the toe boards.
I myself did the "full-length" pans...and the "floorpan-to-firewall extensions" which are really the lower(angled) half of the firewall itself....so its been a tedious process. I am actually finishing up the driver-side tomorrow and probably have close to 20 hours in the job total(all 4 patch panels). It would probably take half the time to install a full firewall and full floor pan(at least if you had the cowl out)

P.S. make that 30...I installed torque boxes at the same time as the extensions.
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