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Old 01-15-2013, 04:20 PM   #1 (permalink)
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Default Why lap?

What is the reason people lap weld instead of butt weld when putting in floor pans? Is because of strength, skill or just easier? I even seen an article in mustang monthly where the lap welded the floor pans in. I mean to me butt welding you could clean the welds and it would look good.
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Old 01-15-2013, 04:23 PM   #2 (permalink)
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What is the reason people lap weld instead of butt weld when putting in floor pans? Is because of strength, skill or just easier? I even seen an article in mustang monthly where the lap welded the floor pans in. I mean to me butt welding you could clean the welds and it would look good.
We spot welded mine, so I cannot contribute to your query.
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Done
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Old 01-15-2013, 04:30 PM   #3 (permalink)
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Did yoh do the one piece or half pans? As far as i know most people spot weld 1 piece floors.
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Old 01-15-2013, 04:32 PM   #4 (permalink)
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Did yoh do the one piece or half pans? As far as i know most people spot weld 1 piece floors.
1 piece...
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2007 Premium MustangCoupe (V6).. Please don't shun me

Done
Replace Full floor pan
Replace Firewall
Replace Full cowl
Replace Radiator support
Shelby Drop done
Front disc brakes on

Up next:

4 wheel drum to disc conversion, rebuild front suspension/steering, taillight panel, trunk floor,302 and AOD upgrade, wiring harnesses back in, and hopefully only patch panels on the quarters

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Old 01-15-2013, 04:40 PM   #5 (permalink)
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Spot weld and used the pieces and now wished i used the one piece
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Old 01-15-2013, 04:47 PM   #6 (permalink)
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I lap and plug welded mine. Punched holes down the rocker side of the pan and the tunnel side then set it in there. Sheet metal screwed it all together and plug welded up the holes. Then I stitch welded the seam between the plug welds together *still have to finish that. ran out of weld gas*. I'll eventually weld the underside of the pan to the tunnel also. Have pix in my link below.

It was much easier to have the single pan set in and level that way compared to lining up the pieces for a butt weld. Also, if I burned through the tunnel I had a lot more material to work with/weld to around that area. To me, and this may just be a matter of opinion, a plug weld that then has a the seam welded is a bit stronger vs just butt welding two next to each other. Honestly do not care what it looks like under carpet and a seam on the underside does not bother me. I'm building a driver, not a show car.
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Old 01-15-2013, 05:11 PM   #7 (permalink)
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To answer your question, most people will lap weld floor pans because it will be under the carpet (inside) and unseen or covered by undercoating on the outside. It's far less effort to lap weld than butt weld. You will see butt welding when you want to make a patch invisible.
As a side note, a lap weld will leave the metal the same thickness (except where it is stacked) while if you are not careful on a butt weld you might grind/sand too much of the weld area and make the metal thinner in that area.
A lap weld is easier to make stronger while a butt weld is easier to make weaker.
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Old 01-15-2013, 05:19 PM   #8 (permalink)
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A butt weld will not trap moisture between the layers and rust like an overlap will.
An overlapping weld is usually faster and requires less skill.

You can overlap and tack weld both sides, then run a body saw or cut off wheel or even a plasma cutter right down the middle of the overlap and have the gap you need to make a butt weld, they even make special clamps to hold it in place.
I would rather have a butt weld with none of the welds ground down than an overlapping weld with goo sprayed over it by the guy that wants to hide it.


If the car matters use a but weld and leave the overlapping spot welded seams where the factory put them in the first place.
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Old 01-15-2013, 05:52 PM   #9 (permalink)
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Quote:
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A butt weld will not trap moisture between the layers and rust like an overlap will.
An overlapping weld is usually faster and requires less skill.

You can overlap and tack weld both sides, then run a body saw or cut off wheel or even a plasma cutter right down the middle of the overlap and have the gap you need to make a butt weld, they even make special clamps to hold it in place.
I would rather have a butt weld with none of the welds ground down than an overlapping weld with goo sprayed over it by the guy that wants to hide it.


If the car matters use a but weld and leave the overlapping spot welded seams where the factory put them in the first place.
That's how it is, IMO
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Old 01-15-2013, 06:46 PM   #10 (permalink)
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If you mean replacing a few toe panels or rear seat footwells, yes, a well performed lap weld is stronger than a butt weld. It is also much faster because you don't have to trim and fit and grind them perfectly. You do twice the welding because you should weld the lap inside and outside.
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Old 01-15-2013, 07:47 PM   #11 (permalink)
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I am a union sheet metal worker so I have alot of experience welding thin gauge metal. The simple answer is that over lapping the metal helps take the heat away. It doesnt matter how good a welder you are sheet metal will twist when heated. Butt welds in sheet metal can be done however they take a very long time to make the almost unnoticable. However you can use a pexto roll forming machine that will jog the patch panel to take your heat but only will overlap minimally. Now that being said if I were doing a floor in a mustang ,it has been quite a few years, I would and have done it like someone else said and punch holes and replug weld. Then seam seal and be on your way. If doing a patch on the body I would jog the patch then stitch weld the fill sand prime paint. Let the criticism start now.
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Old 01-15-2013, 07:54 PM   #12 (permalink)
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.... I would and have done it like someone else said and punch holes and replug weld. Then seam seal and be on your way.....
Makes me feel better about mine! If that's good enough for a guy who does it for a living... it's good enough for me.
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Old 01-15-2013, 08:50 PM   #13 (permalink)
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Well, I can see the next generation of classic mustang owners bitching about how the "Previous owner" jacked up the car with overlapping welds and covered them with "Goo"!

Do you really think they have discussions like this over on a GT40 forum or on some of the high end British cars? Why do we insist on saying it's "OK" to do a hack job on a mustang?
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Old 01-16-2013, 06:52 AM   #14 (permalink)
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I decided a long time ago to do the one piece floor just was wondering this.
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Old 01-16-2013, 10:49 AM   #15 (permalink)
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I don't think this really applies to the one piece pan, just the patches.
Simple enough to me. Lappping is a heck of a lot easier but a bit cheesy. If you butt weld you can make the repairs about invisible and much less prone to long term rusting. That said, pan patches are a pain. I have to admit I used the patches and did my best to butt everything but there are some laps. I hate to see crappy welding on anything and work with it in mind that I wouldn't want someone else looking at such stuff later on and thinking "what jackass did this?" If I had to do it over I'd use the one piece pan and spot it in like the factory did.
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