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Floor pans- flange/plug weld or butt weld?

bigred88

bigred88

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To be clear, these floor pans are for my AMC, not for my K10. And yes I'm on an AMC forum, too, but I feel like you guys have more fab experience than them (and the Built Not Bought spirit is not as strong there), and honestly I don't think the exact car matters all that much for this question.

I'm building new floorpans for my AMC. No one makes after market ones, I guess I could try to find a donor car but I don't want to- it's a good chance for me to learn some sheet metal skills so I'm going to fab my own. I feel like I have a decent plan moving forward, but I can't decide between leaving a flange so that I can just plug weld them in, or butt welding all the way around. I'm leaning toward butt welding, but don't have a good reason for it. What do you guys prefer?
 
I would think a flange and plug weld would be stronger than a butt weld long term and would look better. But I've got no experience with it personally.
 
Depends what the factory did. I butt welded everything that wasn’t supposed to have a step at the joint. Any parts where a flange/step was ok, I did that.
 
What is your intention with the car? Factory restore, nice looker or a horse power monster? How good are the areas you re welding to? What are your welding skills? Even with a flanged, I would fully weld it to seal it and put rigidity in it.
 
ktmoutfront said:
What is your intention with the car? Factory restore, nice looker or a horse power monster? How good are the areas you re welding to? What are your welding skills? Even with a flanged, I would fully weld it to seal it and put rigidity in it.
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I agree with all this and I will say when I am building a car or truck I am building for myself and if I am I will flange when possible and fully weld both sides. And I will make ribs where possible to stiffen large flat areas
 
The car is a driver. Nothing but a fun car for me. Not a horsepower monster, it has a straight 6. I don't really do the show circuit- I'm not opposed to a Cars and Coffee type show but I haven't even gone to one of those in a while. I do try to keep it clean though, and it's in good shape (other than the floor pans, apparently).

My skills- eh. I can weld thicker stuff. I'm inexperienced with sheet metal. The flange seems easier, for sure, and let's he honest, they're floor pans that are going to be covered and I'm not putting any mirrors under it if I take it to a show. I guess I worry about the possibility of rust forming inside the flange. I've had the car nearly 20 years, and I would like to have it for another 20 or more. I don't want to do this again. That thought makes me lean toward butt welding, even though I know it will be more work and a steeper learning curve for me.
 
in that case, I would step flange the majority of it.... throw a bit of weldthru primer in the sammich first... just make sure after ya burn it in, and put some primer on, you go back and seamseal everything... than slather on whatever topcoat ya want...
 
Welding 360 degree is always more work than drilling holes for plug welds, but it's much stronger. You have to fit the parts better for butt weld, which is hard because things will shift a bit as you weld. The butt weld is more likely to shoot through, so you have even more work to do. Then the other side of the panel looks like a porcupine from the extra wire popping through.

You can weld the edges on the flange continuously for full strength (without plug holes) but having a piece of sheet metal behind makes it easier.
 
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