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69ChevelleAddict 05-01-2008 12:48 PM

Butt welding vs Spot welding
I'm getting ready to do the floors of the 71 Chevelle and I have a question.
There is an area that I need to attach the floor to the rest of the car and the factory did it with spot welds.
Because it is so tight in that area I can not get in there to weld.

Would it hurt anything to take a panel that was originally spot welded and re-do it using a butt weld around the perimiter?

F&J 05-01-2008 01:15 PM

I think you mean a lap seam, rather that a true butt weld? That's the way most people do floors on those...lap seam.

69ChevelleAddict 05-01-2008 02:32 PM

I'm leary of lap welding body panels, the guy before me lap welded quarter patches and they are TERRIBLE.
It seems like a lap weld would be a perfect spot for water to collect and rot out.

NEW INTERIORS 05-01-2008 02:40 PM

I just butt weld, :thumbup: No lapping here. :nono: Lapping cause's problem's later. :nono:

cyclopsblown34 05-01-2008 02:44 PM

Use steel rivets and good silicone. The kind of silicone that smells like vinegar will help your car rust away.

lets cut it up 05-01-2008 03:37 PM

if done correctly, lap welds will work on floors. Sometimes you need a backing when doing this type of work and a lap weld will work in those situations where a butt weld will not work. When I have to do it i leave only like a half of an inch to over lap. JMO Tim

F&J 05-01-2008 03:55 PM

err..., lap welds are all over that car, and done by GM ;)

I end up trimming the pans to where they fit the best, and lay them in to trace around them . Then, pull out the new skin and trim the old floor to 1/4" lap. That 1/4" makes a heat sink when welding, plus it gives support when you need to persuade some of the new pans edges to fit tight.

If you think you are up to the frustrations of butt welding an entire floor half...go for it :drool:

But I want to see the results :D

MARTINSR 05-01-2008 04:17 PM

Like I aways say when this subject comes up, the ENTIRE car is held together with lap and pinch welds! There isn't one single butt weld on the car from the factory. EVERY single panel, including the floor that we are talking about is held on with lap welds! Why in the world is one more so bad?

The only reason a lap weld would perform any different than the hundred and fifty other ones on the car would be if it was done incorrectly.

Leave about a half inch lap like the rest of the cars lap welds have.


adtkart 05-01-2008 04:49 PM

If you are going to replace floor pans, atleast do it right. If you think that your but welds are that strong, you better think again. You have to have the panels fit perfectly to get a strong enough weld, specially for the floor. The factory lapped the joints and sealed most with seam sealer. Since most people don't have access to a spot welder to do them exactly like the factory. The best bet is the use of plug welds, done with a mig welder. Apply weld thru primer first, and seam sealer after you are done.

I would not have a car that someone had installed floor pans by but welding them. I would be cutting them out and replacing them immediatly.


BMM 05-01-2008 05:53 PM

Would weld-thru primer and seam sealant on the finished weld not solve the water and rot problem?

NEW INTERIORS 05-01-2008 06:24 PM

1 Attachment(s)

Originally Posted by F&J

If you think you are up to the frustrations of butt welding an entire floor half...go for it :drool:

But I want to see the results :D

Here you go!!!!! :mwink: Some results!!!! :thumbup:

baddbob 05-01-2008 08:05 PM

If the pan you have is the same as a factory pan then definately seperate the seams and install it like factory with plug welds. But if you're using just patch panels then trim them to fit and butt weld them in for an invisable repair. I have had to install some pans that weren't good copies of the originals, a few have even had the rocker flanges pointing upwards instead of down like original-on these I cut as needed and installed only what was necessary with butt welded seams to keep the original look. It's your car-do it the way you want. Bob

69ChevelleAddict 05-02-2008 05:20 AM

I guess im asking if there is a structural difference between spot welding vs a full bead of butt weld.

baddbob 05-02-2008 05:58 AM

The structure is only as strong as the metal it's made of, the factory seam has a flange that's bent and adds some rigidity but if your weld is good the strength will be as good as the sheetmetal itself. The 71 Chevelle is a full framed car so I see no problems with doing a butt weld on the floor anywhere.

F&J 05-02-2008 06:08 AM


I guess im asking if there is a structural difference between spot welding vs a full bead of butt weld.
That's sort of a loaded question in this particular situation. In my opinion, for most backyard hobby guys lacking tons of experience, I would go with the lap on those floors.

I did do floor halves on a 68 Chevelle factory 327 4sp. As I recall, the halves fit pretty good, BUT.. you can't expect any repro panel to fit exactly the same as an OEM mint floor panel cut out of a donor car. That repro one would lead to serious problems when trying to get both the car & panel, trimmed EXACTLY for true butt welding. That's why I said "show me how it comes out". I can't imagine trying for a perfect trimming on a full floor half from front to back. These repro pans may sit a little low or a little high in certain spots when you try to line up the sides of the pan where they curve up. Like I said, I keep fitting, marking & trimming both the body and the pan, until I get a decent fit. I may pull the pan out a dozen times for more tracing & trimming. Then you still need to force certain areas of the new pan to meet those curves.

It won't hurt anything to try to start fitting and trimming for a butt weld, but as you get into it, I think you will see what I mean. The pan will fit terrible before you get the trimming somewhat close to an inch overlap. Go for an "inch overlap trimming" to start with, then see how things line up. If it fits really good, trim to a 1/2" then 1/4" lap. Then you will need to make up your mind to either trim to butt, or stay with a slight overlap. I would never butt, but that's me...I'm old, and I need to spend my time on more important areas...that actually show :D

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