MGB - floor pans (Part 3)

Continuing the effort of replacing the floor pans. In Part 1, I talked through decisions about parts and methods. In Part 2, we removed the old pans. Today, we focus on prepping the new pans and getting them welded in.

Context

paint from below, p-side

Remember that the original pans were spot-welded to the rails. This was done at the factory where very long spot-welder arms are available and things like the doors, dashboard, etc aren't yet installed, so there are fewer obstacles. Even with some crazy-long spot welder extension arms, you will not be able to avoid another solution for some of the mounting locations. Your options: seam weld from below or plug weld from above. At least on the right side, the wires, tubes and cable make a seam-weld from below option very difficult. Most do-at-home folks go the plug-weld route. The folks at British Auto Works in North Plains run seam welds around the outer edge from above, but plug weld the inner frame rails. If you're going to do this, they recommend running beads that are about an inch long and space them every couple of inches to prevent warping. It will hold at least as well, if not better, than plug welding the whole thing.

Fitment

p-side pan prep

The passenger floor fit like it was supposed to fit. It literally dropped right into place. It wouldn't shift too much side to side or front to back, but my hammer/chisel work bent some of the edge along the transmission tunnel, so I had to work the metal back into shape. Once the floor was stable (no wobble or weird clunks from my stepping on it), it was ready to go. The driver (left) side, however, did not fit right. The front right corner on the pan assumed the floor had a squared-off point to it, but my floor had a rounded edge. I had to bend the corner of the pan to test fit and then cut it off with my grinder for welding it in. It would not naturally sit flat either, so I had to be extra careful as I did my first few welds to get the floor to hold flat. You may experience a similar situation, even with the OEM pans. We know the aftermarket ones don't fit without cutting, so be advised.

What's a "Plug Weld"?
A plug weld is performed by drilling a hole through one material, setting that on top of a second piece of material and welding the two pieces together through the hole. In our case, we are making 1/4" holes in the new flooring. The weld is formed by setting the MIG wire into the center of the plug hole, pulling the trigger to start forming a puddle and then extending the puddle into the floor after the puddle has started to grab into the frame rail. Before we can weld, we need to poke holes in the floor, but where? I tried 2 methods. I'll explain them next.

Option 1 - Paint and Measure
Once the frame rails are clean, you just can't help yourself. You just have to drop the new floor on there and admire how nice it looks. It looks so pretty. Looking at the passenger-side pan sitting there, I wanted an easy way to spot the holes. I fiddled with the pan to make sure it was exactly where I wanted it and then set some tool boxes on top so it held still and firm on the rails. Then, I grabbed a can of white spray paint, slid underneath and spray-painted the edges of the frame rails where they met the underside of the new pan. When I lifted the pan off the rails, there were clear white lines showing where the frame rails were. I ran blue tape along those edges and marked drill holes every inch or so with a sharpie. The p-side prep picture on the right shows what the prepared pan looked like.

Option 2 - Use Old Flooring

using old floor, d-side

When I removed the driver (left) side floor pan edges, I didn't crumple them up as I went. Instead, I would separate a section and then cut it free with tin-snips when the trim started getting in the way. Once all of them were free, I arranged them on the ground with the larger sections I had cut out earlier. With the puzzle so completed, I could see where the old spot welds were. So, I arranged the puzzle onto the bottom of the new pan, and shot spray-paint through the taped-on floor-pan edges. When I removed the old edges, I had dots showing me where to drill the plug-weld holes.

Drill Baby Drill
Regardless of which method you choose, drilling the plug holes is a two-step process. First you drill a pilot hole and then you drill out to 1/4". I suppose a step-bit would work so that would be only one drill effort. Similar to cutting out the spot welds, this is time consuming. I understand why British Auto Works (and probably most other shops) reduce costs by seam welding the outer edges. For that reason, I drilled plug welds into the floor to correspond to the inner frame rails, and only a few around the outer edge of the p-side pan. My paint-from-below method worked pretty well, though my alignment slipped a little bit and I had a few holes that didn't align with the rail.

On the driver (left) side, I chose to plug weld the whole thing, so that meant more drilling on that pan. The holes I produced with Option 2 were an exact match. They aligned on the rails perfectly. Unfortunately, the holes that cut all the way through the frame rail were now completely exposed as well. The drilling out of a single pan took me 2-3 hours and overall there really wasn't that much of a difference in time between the two sides, even though there were far fewer holes on the p-side. That doesn't math-out logically, but that's what happened. The all-in time at this point is over 14 hours and no pans are welded in yet.

Plug Welding

If the grinding was the best part of Part 2, then the welding was the best part of Part 3. I borrowed a 110V Lincoln MIG welder from my wife's sister's boyfriend, and bought a small 20# tank of "steel mix" gas for the project (cost me $100 for the tank plus gas-fill on craigslist). I've heard these small tanks have 6-8 hours of gas in them, so that should be enough for multiple pan replacement projects. Once I had the passenger side ready to weld, I hosted my wife's sister and her boyfriend for a welding party. The plan was simple: he shows us how to weld, and I keep a fresh beer in his hand. It worked out well for everyone. While he got the welder set up, he taught us with welding scraps from the old torn-out floor. This gave us opportunity to work on pieces that didn't matter, but were the right thickness and material. We ran the temp / voltage as high as it would go (D) and the feed set pretty fast too (just below 9). Because if the thickness of the rails, the high-voltage setting makes sense. For other work, like repairing the rear bulkhead the previous owner cut a big hole in, I'll need to bring the temp way down. Before we were done for the evening, the passenger pan was in, and 3 of us learned how to MIG weld, including my wife's sister. Very fun.

During the actual weld-in, we had setbacks, of course, like some burn-through of the pan when we novices tried to run a bead, or small fires from grease or paint catching fire from the welding. Not to worry, though, they were small enough to blow out. Still, the decision to weld on the driveway rather than the standard low-ceiling garage turned out to be a good idea. We tested a few of the welds just for curiosity sake by banging on them with a hammer and trying to separate the panel with pressure from below. No dice.

I've gone back and forth about grinding down the plug-weld humps. I know the floor is going to be covered with sound reducer, a carpet pad and then carpeting, so no one would ever really see them. At this point, though, I'm trying to get the car on the road before the weather turns, so maybe I'll grind the humps down under a canopy once the weather changes this fall. Or I may just leave them alone, and focus energy on value-adding efforts, which grinding down weld-humps in an area no one will ever see... frankly isn't.

That's it for today. My final steps are seam-sealing the floor edges, and getting some paint or undercoating on the steel pans. That work may not be interesting enough to warrant a post. After that's done, I am going to focus on getting the fuel system and master cylinders re-installed. Expect posts on those efforts soon. As always, thanks for following along.