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To butt weld or not

Discussion in 'The Hokey Ass Message Board' started by captaintaytay, Sep 22, 2012.

  1. Hi, its been awhile since I've been here. I'm going to be patching in passenger side floor pan on my 50 shoebox and was curious if I should butt or overlap weld?.
    I had to cut out previous floor pan as it was a bad job from previous owner.
    I have a Lincoln 140 Mig Welder and I'am a novice welder (couple of years), or do I need to tig weld it? as I've read. I'am enclosing some pic's. Thanks as always for your advice.



  2. May Pop
    Joined: Jun 16, 2005
    Posts: 125

    May Pop

    Butt weld. Then grind smooth. Lap joint looks sloppy and is a place for rust to form again.

  3. Butt weld is so much better.

    The mig overlap is easier to do and is a great place for rust to start again.

    Spend a few more hours and it will last a few more decades longer - how's that?
  4. Well thank you guys for the quick response, I will be butt welding and just to confirm, mig welding this job is okay?

  5. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,483


  6. Yep mig will be fine. Just get a good fit up - both pieces flush to within 1/2 material thickness & with about 1 material thickness gap. Too much grinding is never good.
  7. Yes that will be the key good straight cuts so it fits proper. Don't want grind any more than I have to. Thanks again and I will post up pic's after floor pan is installed.:D
  8. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,483


    Take your time. Weld a little at a time, and allow for cooling. Warping is not fun. It is not an exterior body panel, but it is good practice for when you are going to do one.
  9. Cerberus
    Joined: May 24, 2010
    Posts: 1,392


    Sparks will fly. Protect your dash and glass and ass, etc. I use 3M welding and spark deflection paper with adhesive backing when I weld inside a vehicle. Also have hi-temp welding blankets to throw over seats if necessary.
  10. davidbistolas
    Joined: May 21, 2010
    Posts: 962


    I lap welded mine, but it's a really close lap - 1/4" maybe. I don't know what I'm doing, so follow the advise of the smarter ones here. :)
  11. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,483


    Lap welding works. Just seam seal both sides immediately, to avoid moisture inclusion.
  12. choptvan
    Joined: Mar 19, 2010
    Posts: 2,161


    Even factory floors are lap welded. The only problems with moisture getting in the seam, is where the seam sealer had broke free afrom feet rubbing itand it allowed the water to get under it. I usually lap weld it by 3/4 inch with plug welds. Clean the welds. Seam seal it. Then rubber coat it. This is both top AND bottom.
  13. a good trick for speed and ease on floorboards is to over lap your patch panel 1/8" or less and as you weld melt the edges together so you end up with no overlap. this will give you a step the thickness of the sheet metal which you can hammer even if you want to after grinding.
  14. jw johnston
    Joined: Oct 16, 2011
    Posts: 106

    jw johnston

    Lay youre new panel on the area its going. Take a scribe and trace around the panel, then cut, grind and file till you get a good fit . Clean area around the floor to be welded to bare metal top and bottom as best as possible. Clamp. Then weld With mig, gas or tig. Just take your time welding and grinding the welds like said be fore its not and exterior panel, but i have seen people really screw up a floor warping it.
  15. The interior is pretty much gutted, but will protect dash as suggested. Thanks

    Yea, Thats the main reason I'm replacing pan as PO warped it causing clunk every time someone got in or out of it. Thanks for your help.

    I'm a little torn now as some say lap weld and some say butt weld, I have no choice though at rocker as it can't be lap welded so I'm going to butt weld the whole thing. Thanks to all for the great help. I will report back.
  16. davidbistolas
    Joined: May 21, 2010
    Posts: 962


    Any advantage to fibreglassing the inside of the floors for additional protection?

    ...I mean after welding is complete...
  17. at the rocker you can put bend in the patch panel to make welding easier.
  18. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,483


    No. Never seen that help, only harm. Primer, seam seal, paint, undercoat (paint-able variety) paint again, both sides. Allow adequate drying time between all material coats. Have stuff in New England, and other Eastern states (from when I lived there) holding up just fine, after 25+ years, using this method. Including winter-drivers.
  19. LUX BLUE
    Joined: May 23, 2005
    Posts: 4,407

    Alliance Vendor
    from AUSTIN,TX

    No. The constant heat changes, foot traffic and mosture will simply make the fiberglass lift up and become the perfect water trap.

    If it is re-enforcement or sound deadening you want, or if you have floors that are thin but not wasted, clean them a put down a single layer of fiberglass mat, but instead of using resin to glue them down, use por15. It works really, really well.

    Sent from my DROID device using the TJJ mobile app
  20. 444
    Joined: Feb 17, 2011
    Posts: 15

    from Canada

    Lap weld for anything structural and/or hidden. If you prep the metal properly before and seal it properly after, it will be fine. On the body I prefer butt weld because it's easier to hide the seam than a lap weld.
  21. Finnrodder
    Joined: Oct 18, 2009
    Posts: 2,965

    from Finland

    I've replaced most of my shoebox floorpans.I butt welded them and welded by mig.Welding clamps are really useful with this job.Good luck!
  22. Update:

    Hello Guys, I made some time to finish floor pan repairs and it came out pretty darn good.
    Passenger side floor pan in flat, level and no pop.
    Took some pic's not the best, sorry.

    Next is to fab some seat brackets and some kind of quick release mount for fuel cell that is in trunk. Will start searching.

    Again, Thanks for all the help and advice.


  23. manyolcars
    Joined: Mar 30, 2001
    Posts: 8,707


    finish those tack welds or moisture will rot out your welds
  24. Da Tinman
    Joined: Dec 29, 2005
    Posts: 4,227

    Da Tinman

    if there were no factory lap welds 80-90% of the rust we deal with wouldnt exist as there would be no place for it to start. I eliminate them everywhere I can.
  25. I was under the impression that all I needed to do was primer, seam seal and then undercoat both sides.
  26. Overtime
    Joined: Oct 22, 2012
    Posts: 47


    I would weld solid, then do as mentioned
  27. 49ratfink
    Joined: Feb 8, 2004
    Posts: 18,271

    from California

    weld it in solid. I assume you didn't butt weld?
  28. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 28,929

    Jalopy Joker

    butt or lap - i am so confused
  29. hoggyrubber
    Joined: Aug 30, 2008
    Posts: 572


    i have done the last few floor pans butt welds with ox/acet and have been pretty happy with the results. it's quite a bit slower than the mig, but easier for me to control the heat. another thing it getting the piece cut just right takes a lot more time. i have done lap welds on the trucks i have in the past- faster but uglier.

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