I am fitting the RD front and rear floor pan pieces for replacement.
The front and rear overlap. I am assuming I should fit and trim and then butt weld them together.

Would there be any reason to overlap and lap weld them?

Thanks

-Andrew

I trimmed them and butt welded them together; in my mind it makes for a cleaner installation. I'm not sure it makes that much difference. In theory overlapping would be stronger if you weld both sides

http://www.914world.com/bbs2/index.php?s=&...t&p=2422705

I’ve seen both but if you lap weld, make sure the front is below the back. If you run over something you don’t want to catch the lip.

Butt weld. It's worth the effort.

If there is no significant strength benefit to the lap I would prefer the stock look
Thank you
-Andrew

Lap = traps moisture.

It depends on the car, and your skill level as a weldor. A driver quality car would be different than a high quality restoration.

If you don't have a true gas MIG, then don't even think about butt welding. If you try and butt weld with flux wire, you'll have weld inclusions and gnarly welds.

A production body shop would typically want to do a lap joint. When you flange the panel you do add some stiffness, it's much easier to weld, and you have a little more margin of error fitting the pieces. For a pan, I'd want the overlap seam in the cabin, not visible under the car, or to let moisture in the lap seam.

If you are a moderately skilled home welder with a good MIG (not flux coated) this is what I'd recommend. MIG welds are lower in tensile strength, and butt welding takes more skill. If you end up with any trouble with your butt welding you can have a much weaker joint that's more prone to crack.

Part of it is where are you putting the lap joint and how critical is appearance vrs strength.

MIG butt welds on body panels work better when you have a small uniform gap between the two panels, which can also make it easier to blow through. If you don't have a gap, you'll have more warpage. Whereas TIG is better if you're panels are perfectly butted together and you aren't filling the gap, which is virtually impossible on a large panel. This isn't to say you can't fill gaps with a TIG, it's just much harder than a wire feed MIG.

The prince of welding technique is TIG, but it's very time consuming and technique oriented; not newbie friendly at all. Correct TIG welds are much stronger than MIG and have a higher tensile strength. Metal condition and cleanliness for TIG is several notches above what you can get away with on a MIG. A restoration shop with craftsmen would probably prefer to do a butt weld with TIG, but it takes alot more time, and even then in some cases not practical. The optimum filler rod for TIG on a floor pan would probably be .035 MIG wire

I've been lucky with the few pans I've had to work on. The metal was good in the tunnel, and I used left and right pan sections, not the front/back pieces.

The car was on a rotisserie, so it made it much easier and less welding out of position. I cut the bad pans off along the center of the weld nuggets along the flange for the tunnel, leaving a ledge to set the new pan on in the center, and then plug welded along the longitudinal. It took careful work with a cutoff wheel and carbide burrs to get the pans off, but it make for a very good repair. I'm a fair TIG weldor, but not competent enough, and not willing to fight any metal prep issues with an old lap flange, I used MIG.

I am not good enough with my TIG on thin sheet metal. I got good back in my metal sculpture days TIG welding heavier stuff, but never dealt with thin stuff. I guess I could practice with some 18ga a bit before I do the work on the car—Ill pull it out and try that.

The car is on a rotisserie so I figured if I got the seam pretty tight and backed it with a copper plate and took my time with my MIG I would be OK.

My butt welding skills are getting better as I patch things here and there, but I still have trouble now and then when I am working with areas that I have had to blast a lot or clean rust off —areas that while solid tend to have some thinner spots.

These are new Restoration Design panels so I think if I take my time I can get a good weld with my MIG.

Thanks for the responses

-Andrew

My MIG welding skill are mediocre at best, yet my floor pan replacement turned out really well. Like you said, welding those nice clean and solid RD panels is a cake walk compared to rust repair in sketchy metal.

I am just finishing up a a Notchback that had a lot of lap joints and all were full of rust. The car had never been on the road again but still rusty.
I trimmed everything and butt welded it all.
It was worth the effort.

Agree

I'm gonna have to do the rear floors on my car, and I agree about butt welding ... and while that is easy enough from the interior side - are you guys (without a rotisserie) MIG welding, butt welds up-side-down, from under the car? Plug welds along the edges, for that matter? I consider myself a descent MIG welder, but haven't welded much while laying on my back and a car on jack stands. Am I over thinking it?

Somewhat counter-intuitive, but butt welds are actually stronger than lap welds due to the fact that lap welds allow rotation and add a combined load to the weld/HAZ.

Yup. With the right settings and some practice its not too big a deal. A bit harder and less convenient certainly, but totally doable. Much better than out of position TIG welding for sure.

Yup, I’ve done a few upside down. Not the greatest approach but doable. The hot lava falling on your skin is a real treat.

If this was a modern car it would be a lap joint that was glued together

Thanks guys.

I've actually considered cutting the top of the tunnel shell off (in a strategic area) in order to access the weld seem. In theory It'd also allow better access for treating it's interior for rust treatment, clutch tube re-inforcement, etc. - before welding the tunnel section back on.

Anyone gone that route?