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Floor Pan Replacement: What's Underneath?


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One of us is going to learn how to weld!!  I thought I would only have one small repair and could panel bond it.  No dice.  I think the prudent thing to do in my case is to replace both front floor pans, pretty much completely.

 

So I have a few questions (and I won't be able to investigate under the car until this weekend):

 

1)  I'm going clean the top of the area to be welded down to bare shiny metal - what's on the underside and do I need to worry about it?

 

2)  The outboard driver's side seat mount seems to be spot welded to the body.  Am I correct in assuming that if I drill out those spot welds that the mount should come off relatively easily?

 

My "plan" is to soda blast one side at a time, see the extent of the rust and then use a combination of sand blasting and trimming to get back to good metal.  Then I will re-assess how big a patch panel will get welded in.  Does this sound about right? 

 

I think this will also be a good time to inspect the frame rails too, right?

 

This will be my first welding project - and first real task on the 1600-2.  Any and all suggestions welcome.

 

Thanks,

Bruce.

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You're gonna have to remove the tarpaper coating from the floorboard's inside to see the extent of the rust--it hides most of the metal on the inside, and you'll need to determine just how close to the outer frame rails the rust extends.  You really don't need to clean down to shiny metal until you're ready to weld--an ice pick or old screwdriver will tell you pretty quickly where the rust ends and sound metal begins. 

 

You want to cut back to clean metal--weld doesn't stick to rusty metal very well at all.  No need to sandblast--that'll make an awful mess inside your car.  Use a grinder.  

 

Presume you'll be using a MIG welder, not a torch; it's tough enough to keep from warping thin sheet metal with a MIG, much less a torch.  When you do get ready to do the deed, remove your seats, console and carpets, cover the dash, door/quarter upholstery panels and package shelf with wet towels, and use something to shield the headliner--weld sparks/spatter will burn holes in it in a heartbeat. And make sure you're well away from the fuel line that runs along the base of the pax side inner frame rail (inside the car) and the steel brake lines under the car.  And have a water bucket with a wet towel in the bucket and a fire extingisher close by.

 

If you've never welded before, suggest signing up for a class at your local vocational high school's night division--it's money well spent to learn the basics. 

 

Good luck.  Welding up rust/damage yourself is a very satisfying thing, so long as you don't burn yourself or your car!

 

mike

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I bought the car as a non-running shell.  The prior owner started restoration but ran out of steam.  Right now there is no interior except for the headliner, which badly needs replacing.

 

I used dry ice to get the sound proofing out and then I beat the heck out of the floor pans with a pointy body hammer finding soft spots.

 

Maybe my question is more about general welding:  I know to clean the weld area down to shiny metal; I see them do that on every car show but what about what's on the backside of the floor panel - do I have to worry about that bursting into flames or smoldering and killing me with fumes?

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One of us is going to learn how to weld!!  I thought I would only have one small repair and could panel bond it.  No dice.  I think the prudent thing to do in my case is to replace both front floor pans, pretty much completely.

 

So I have a few questions (and I won't be able to investigate under the car until this weekend):

 

1)  I'm going clean the top of the area to be welded down to bare shiny metal - what's on the underside and do I need to worry about it?

 

2)  The outboard driver's side seat mount seems to be spot welded to the body.  Am I correct in assuming that if I drill out those spot welds that the mount should come off relatively easily?

...

1)  Once you have cut out the rusty metal as far in as you need to go to find sound, rust-free metal, if you can access the hole from the underside, you should also grind that perimeter down to bare metal.  That will avoid any possible contamination of the weld puddle and will help to avoid any of the coating on the underside from catching on fire or throwing off some nasty fumes.  This is not critical, but it helps.

 

2)  You are correct.  If you drill out all of the spot welds, the piece should come off cleanly with very little effort.  Note that there is a special tool, appropriately enough called a "spot weld cutter" that is specifically made for this task.  The advantage of using that tool is that it is designed to just cut the top piece's spot weld with cutting (or drilling) all the way through both pieces of metal.

 

Regards, Maurice.

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  • 6 years later...

I would like to piggy back in this discussion and see if I can get my questions answered.

 

I brought my car to a new shop because my brake was not working.  I basically do not have any braking power.  When the mechanic finally got a chance to look at the car, he showed me that my pedal box is practically attached to rust.  that's why there is so much play on my brake pedal.

 

The recommendation was:

1. Strip the whole flooring from the top to see the extent of the rust.

2. Replace the floor pan and possibly the pedal box.

 

My questions are:

1. Do you have to mess with the engine and transmission to replace the floor pan and pedal box?  Can it be replaced from the top?

2.  Why cant he see the rust from the bottom and why does he need to strip my whole floor from the top to do that?  I hesitate to have that done because I just had the interior done no more that 2 years ago.  If there was rust there, I'm sure the guy who did it would have told me that.

 

I am open to suggestions.

 

Thanks.

JC

 

 

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1 hour ago, JC Orosa said:

I would like to piggy back in this discussion and see if I can get my questions answered.

 

I brought my car to a new shop because my brake was not working.  I basically do not have any braking power.  When the mechanic finally got a chance to look at the car, he showed me that my pedal box is practically attached to rust.  that's why there is so much play on my brake pedal.

 

The recommendation was:

1. Strip the whole flooring from the top to see the extent of the rust.

2. Replace the floor pan and possibly the pedal box.

 

My questions are:

1. Do you have to mess with the engine and transmission to replace the floor pan and pedal box?  Can it be replaced from the top?

2.  Why cant he see the rust from the bottom and why does he need to strip my whole floor from the top to do that?  I hesitate to have that done because I just had the interior done no more that 2 years ago.  If there was rust there, I'm sure the guy who did it would have told me that.

 

I am open to suggestions.

 

Thanks.

JC

 

 

 

1.  You don't have to mess with engine/transmission when you replace the pedal box.  But, you do need to get under the car, as well as inside the car, to remove it.    

2.  The interior of the floorplan is covered by carpet, cardboard then sound deadening material.  That all needs to be removed in order to expose the metal beneath.  Pretty easy to pull all of that up yourself to see what's going on.  Underside is covered in a factory undercoating that can hide rust.  

 

When your interior was redone, did they go down to metal on the floorplans?  Or did they just replace the carpet?  

 

If you were to get under your car and push on your pedal box, does it move?  Can you take some pics of your pedal box?

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According to the mechanic, there is nothing holding the supporting bracket (pedal box), due to the rust.  I will take a closer look at it.  I will take pictures/video of the pedals and the rust.

 

Thanks for the response.

 

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21 hours ago, JC Orosa said:

If there was rust there, I'm sure the guy who did it would have told me that.

 

 

 

 

 

I'm not so sure about that! He could have been just getting the job done and throwing carpet down over bad metal. It is possible, however, that just your pedal box is rusted. Gotta get in there to be sure. You can look at the pedal box and at the bottom of the car to get a rough idea, but they don't call it the rust worm for nothing. It will hide beneath finishes and tar paper.

 

I am in the same boat of having rust to unknown extent in my floor pans. I am about to pull up two layers of carpet and all the junk underneath. I am hoping it's just floor pans and not sills. It looks like my frame rails are decent, but we'll see!

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It's a common rust area, along with the frame that extends under the floorpan.

 

This is what I had to deal with (PO had previously did a bad patch job)  I also had to replace the pedal box, it had a lot of rust holes in it.

 

20180602_181353.jpg

 

I blogged about my repair process:

 

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  • 4 weeks later...

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