Novice question, thought I knew what I was doing, but during my welding course last week an instructor and I were talking cars and he said he did floor pans once in some 40s car and he managed to shrink the car enough he couldn't shut the doors, because he stitch welded the floor in, he said to plug weld it.

I was planning on getting everything all trimmed up with a 1" overlap for a lap weld, screwing the pan in to place every few inches and then slowly stitch weld at random places, keeping everything as cool as possible until I was finished with a nice solid weld all the way around... This should be fine, no?

 

FWIW, The factory spot welded everything in the car, and sealed the gaps with seam sealer.

 

More than likely his case was more of a large floor pan replacement together with lack in bracing during the work. For a small patch, as described would be fine. Butt welding worked fine on my 67

 

These are full floor pans. I'm basically doing the entire horizontal area of the floors from the seam on the firewall to where the bracket is for the rear seat cushion bolt. I do have subframe connectors and the rockers are still in place. I wouldn't think there would be THAT much distortion if I make sure to go at a really slow pace with long intervals between welds... I just don't feel like it will be as strong just plug welding it in to place every inch or so.

 

If the whole floor is cut out, not a bad idea to get some bracing installed to tie the two rockers together while the floor is out. If the tunnel remains in place,. can probably be ok without bracing doing one side at a time.

 

Bracing will only cure the issue of warping if you leave the bracing in place. Once the bracing is removed the metal snaps into the shape it would have been had you not used bracing.

 

The idea for the bracing is to keep the body from twisting while setting the new floor in place. To keep the whole floor from heat warping the slow stitching method should work.

 

Welding the entire perimeter of a replacement pan would be extreme overkill. Aside from potential for warping, it'd be a waste of time and raw materials (mig wire and gas).

Once the seam sealer is cleaned off the joints, you can generally see daylight between the stock pans and inner rockers. Even the structural parts of the car are spot welded.

 

Alright, plug welds every inch or so are the way I'll go.
The tunnel is still in place, I left it in place to make the job a bit easier.

I have some aerospace 2-part "seam sealer" that we use along edges of panels that need to be sealed that I'll be using... Stuff that is expired by a month or two, still good but it no longer is acceptable on aircraft.
Once it's welded in place I will epoxy prime the joint, lay the seam sealer in place, smooth it out and then spray my topcoat on. We paint over our seam sealer stuff all the time.