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MG MGB Technical - floor pan spot weld locations

I am about to remove the floor pans from a 67GT and am having difficulty locating the plug welds. I have a spot weld cutter to do the job.

I bought what I believe to be oem type replacement pans from Moss. Should I assume that the location of the plug holes in the new pans will be the same as the original?
Steve Rechter

Have your new floor pans got weld holes already punched in them? Mine didn't. I counted 107 spot welds on each side of the car and had to drill that number of holes to fix the floors in. See photo.


Mike Howlett

Steve,

Years ago I welded new pans in our MGB tourer. I can tell you to find, and remove the original spot welds may be very difficult. I cut our old pan out of the car along the base of the gear box tunnel, footwells, sills, and rear bulkhead with a nibbler. I cleaned, and primed the old metal. I then used (hammer) steel pop rivits on corners of the new pan to locate and align where needed. I then stitch welded one inch long (approx.) welds with one inch long spaces between welds alternating side to side to allow cooling time. Cleaned, primed the metal along joints, filled gaps top and bottom with automotive joint compond, and then painted with rust inhibitor paint.

Regards,

Larry C. '74 B/GT
Larry C '69 Midget

I just unpacked the pans and Mike you were correct. They do not have holes punched in them.

To my surprise they do have captive nuts for the seats already installed. From what I have read in the archives, it was questionable whether the early replacement pans came with them.

BTW, these pans were made by KAS in Australia.

So should I then assume that the factory stitch welded the originals in place rather then plug welding?

Larry: Did you leave the lip of the old floor pan in place after you cut it out along the base? I take it you used a powered sheet metal nibbler.
Steve Rechter

Cut the bulk of the old floor out with an angle grinder so that you just leave the part of the old floor that is welded to frame. It's a lot easier then to separate whats left of the floor from the frame with a chisel or even grind away any difficult bits.The best way to mark the parts you want to cut away is by drilling pilot holes in the floor from underneath, then join the holes with a line. I hope you can make some sense of this,it's not the easiest thing to describe. Barrie E
Barrie Egerton

Steve,

Yes, using a nibbler I did leave a lip, perhaps a half inch wide. As Barrie points out their are many ways to cut metal including a cold chisel. He also has a good point, and would also recommend drilling holes from below as a starting and stopping guide. Be sure to wire brush clean sorrounding metal, their will be smoke!
Larry C '69 Midget

Thanks guys for all of the acquired knowledge. I plan on cutting them out from the bottom. From that vantage point I feel safer being able to look at the lip along the sills, etc. and not fear cutting in to them.

Did the factory punch holes along the pans to plug weld them or did they stitch weld them?
Steve Rechter

That's what I did too. I cut the majority of the metal away from underneath (car on its side on a roller) leaving the lip. Then I lightly ground the remainder so that I could see the welds. The floors are spot welded in place, 107 spots on each side of the car. I used a combination of drilling out the spot welds, grinding the metal out, and air-chiselling the old flange off the lip. Its a horrible job down in the footwells. The welds go right round the edges and also along both sides of the cross member and the fore-and-aft strengthener under your feet.
Mike Howlett

With 107 welds x 2 to cut/grind it has me thinking.

What danger or problem would I have if I left the old lip attached and just weld the new one on top of it?

The edges on my pans are rust free.
Steve Rechter

I did it how Mike described. Cut out the bulk of the floors then using a sander/grinding disc on the angle grinder went around sanding the bit left. You soon see where the original spots were. I ground them thin then used a hammer and chisel to remove the strips left. If you get it right you can just peel those away.

To make all the holes I used a tool called a joggler. It has a dual head on it, one side a flanging tool for doing lap welds and the other is a quarter inch hole punch. It was well worth it! You still have to drill the holes in the middle of the floor where it welds to the cross-members of course.

There is a pic here: http://www.asciimation.co.nz/pics/page5.html

I figured there were about 120 holes in each floor. Apparently - was so long ago who can remember!

Simon
Simon Jansen

I wouldn't have been happy to weld new floors onto the old flanges. The more pieces of metal you have coming together, the more likely it is that damp will creep in and rust will start there.

Simon, it wouldn't surprise me if your floors had 120 spot welds. After all, the cars were built by men not by a pre-programmed machine. The guys would put in as many welds as they thought right on the day. I found the joggler holes were too small - maybe it was my cheap welder, but I needed a bigger hole (say 8mm) to create a decent, strong plug of weld. If I welded through the smaller joggler hole, I found I didn't always get complete penetration of the lower piece. In any case you can't get the joggler to the centre of the panel for welding over the cross member and footwell support. It is not a job I would do again if I could possibly avoid it.
Mike Howlett

I agree with Mike, the joggler punch hole are a bit small and the plug welds can end up rather marginal as a result. One way to combat this is to turn the power up on your mig to a level greater than you would normally use on that thickness of steel. However, you need to be well practised with the mig.

If you have the shell in a rotator it would be easier to make the holes for the plug welds in the flanges rather than in the new floor. This would work well around the foot well were doing the plug welds from inside would be a pain.
David Witham

There is a trick to welding those small holes. I found I had to start in the middle of the hole with the MIG wire starting the weld pool in the bottom piece (the flange) then spiraling out so it was nice and hot by the time I reached the edge of the hole. Easy to say but hard to do with your head stuck right up in the passenger foot-wells!

These were the second set of floors I had done so I was well practiced by then though.

As others mentioned treat the insides of the cross-members with some anti rust paint before welding the floors in place. I then welded the floors in, seam sealed the underside then literally poured rust proofer in from the top to creep in and fill the seams. I let that sit a while then wiped off the excess and seam sealed the top. That was eventually painted over.

Simon
Simon Jansen

I know there is more than one way to skin a cat, but I don't recommend cutting the floor out as others have recommended. What I did with my 67GT was to use a wire brush and clean the metal about 1 to 2 inches from the edge. I then spray a light coat of white paint around the edge. A light sanding will make the spot welds stick out. Use your spot weld cutter to cut the spot welds - Yep, it takes a lot of work to get all 120 or so cut out, but you end up with a great pattern to use for the holes in your new one.

Once out, I used a grinder to smooth the old welds. Put in the new floor. Put some weight on it in the center of the pan and use a mig welder to fill the plugs. Comes out looking very close to factory.

After welding the floor in, I used a seam sealer around the edges to keep any moisture out.

I also coated the rails with weldable primer prior to putting the floor back in.
Bruce-C

You do have to make sure you press the floor down right near the weld to make sure the floor and flange are in close contact. Remember the heat form the welding can cause distortion so things stop sitting flat. You definitely can't get a nice weld if the floor is slightly lifted off the flange.
Simon Jansen

I remember that Eastwood had some removable rivets that can be used to hold the panels down.

http://www.eastwood.com/autobody/panel-install/3-16-inch-blind-grip-holders-kit.html
Bruce-C

Tack-weld with very large gaps between, then fill in leaving progressively smaller gaps. If you start at one end and work your way round you will end up with more floor pan than there is flange to weld it to.
Paul Hunt

Hi Simon,As Simon knows i'm doing my floorpan as we speak.It a nasty job,however very rewarding when done.I had my 72 B roadster acid dipped and then "ECoated"so all my spot welds were very clear.The rails are very nice and ecoated but i will most likely put POR15 on them too.I'm only plug welding the center parts,and stitch welding the outer edges.They seam sealer.I ordered mine from The Roadster Factory in Penn. All seat nuts were welded on,along with hanger brackets.Mike 8mm you say?Good luck Steve,Regards,Rich o
rich osterhout

You guys are great.

I did as Bruce-C suggested and found the welds to be very visible.

I will be drilling them out with the spot weld cutter this weekend - if the weather cools down a bit. (It has been >100F for the past week.)

I also purchased a hole puncher (9/32") for the perimeter holes and will be drilling out the center ones. Everything I read online said that for structural integrity a 5/16" plug weld is optimum.

(Here is a link to the punch: http://www2.northerntool.com/hand-tools/chisel-punch-stamp-kits/item-14572.htm)

I purchased weldable primer and seam sealer from eastwood.com for a pretty good price.

I will post some pics soon.
Steve Rechter

The weather cooled down and I started on the floors.

When I got to the passenger side I got a nasty surprise hidden under that tar-like floor insulation.

It is rusted through above the toe end of the floor panel.

I've attached a couple of photos. One of the rusted area and one of the project car on the left ('67 GT) and my daily driver ('75 anniversary edition) on the right.

Looking for info on how to repair the area above the floor panel.

Tnx,

Steven

Steve Rechter

Here is the second image:



Steve Rechter

Steven, that area you will need to make up and install patches yourself. It's not too difficult depending on how far up the rust goes!
Simon Jansen

Yes, I had to repair part of that panel. It is basically flat metal so isn't hard to make a patch. Mine had gone round the outer edges (see picture). I cut back to sound metal and made a piece to fill in the gap.


Mike Howlett

This thread was discussed between 25/07/2009 and 02/08/2009

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