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MG MGB Technical - Floor Replacement Correct Procedure?

Hi All

Received the Steelcraft floor pan for my MkI GT. Unfortunately it does not appear that OEM with all the nuts for the seat welded on are available any more. I used the old pan as a template for placemnt of the seat nuts on the new one.

This pan has the lip on it. Some of the archives recommend removing the lip if not needed. Any real reason to take this off? I fitted the pan in the floor opening and it appears to fit very well. Used a grease pencil to mark the support areas on the bottom of the pan.

Old floor lip is in very good shape. All spot weld dots have been ground flush.

In reading the archives, it would appear drilling a 1/4 hole spaced about 1" along all of the weld seam areas of the new pan is the next step and then refit in the car,make sure the pan is right aganst the weld lip, tack at a number of spots and then work around the panel filling the plug welds. Is this correct?

I also sprayed all the support areas with a weld through primer.

Is the above the normal way to replace the floor pan? Any input prior to drillng the pan. If anyone has a good site with pictures, I would appreciate seeing it.

Bruce Cunha

Bruce,

I don't know if there is any one "correct procedure" but I wouldn't hesitate to buy an MG that was repaired in the way you propose. While you're drilling all those holes (and it's much faster working around the perimeter if you use an air punch), fit the floor in place and from underneath, trace around the crossmember and the frame rails that go forward from it. Then you can remove the floor and drill holes to plug weld to the crossmember and frame rails, using these lines as your guide.

You'll need to locate, drill the holes and weld in the captive nuts for the seat rails before installing the floor, as the two forward seat rail bolts go inside the crossmember. If you forget to do this, there is a remedy which I can detail for you later if you need it. I generally set up a seat on the rails on the floor panel while still on the bench, and use that as a template. If you got your old floor out in a sufficiently large piece to use as a template, that is probably easier. I would still bolt a real seat through the holes you've drilled in the floor before welding in the captive nuts. That way, you can test to make sure the seat slides easily without binding. If minor adjustments are needed, you can always drill one or two of the holes a bit oversized, allowing you to move the nut to where the seat moves easily. Then turn the whole assembly over and weld the nuts in place.

I leave the flanges on the replacement floor panel as much as possible. The flange that faces the transmission tunnel isn't always stamped acurately enough to fit snugly, so sometimes I cut part of it out. But I think the flange allows me to do a neater, stronger job - I also weld to the tunnel and inner sill, as well as to the old floor flange on the tunnel and the castle section below the inner sill. This is not only neater and stronger, it's also easier to seal.

Watch out when cutting the old floor out on the passenger side. Take a look at all the fuel and brake lines, battery cable and rear wiring harness running right underneath. It's easy to cut them accidentally. ' Been there... done that. If you're going to take chances, at least disconnect the battery first! And have a container to catch spilled fuel handy. If you cut through both the fuel line and the wiring at the same time, plan an escape route first and have an appropriate fire extinguisher handy.

' Just my 2 worth. Send me your e-mail address and I'll send you a few pictures of what I've done. No matter how artful you are with a MIG welder, MIG welds cannot be made to look exactly like spot welds. In other words, once you hit the car with a MIG welder, it's no longer concours. Just make it as original looking as possible, consistent with integrity, neatness, and strength.

Best regards,
Allen
Allen Bachelder

Thanks Allen

Read over all I could on this process, but as it is the first one I have done, I always want to check my work ahead of time so I don't screw up too bady.

Your suggestion on putting the seat rails on the new pan is one I didn't think of. It's a great one. I was able to use the old panas a template, but double checking with the seat prior to welding the nuts on is a great idea.

Only had to do the driver side so did not have all the issues of electrical and fuel lines.
Bruce Cunha

Bruce, buy a Roper Whitney Jr. hand punch. It's about a $50 tool including several sizes of dies, but it will punch holes very fast and has 1000 uses. You can easily punch 100 clean holes in 5 minutes. If you were to drill all those holes, you would have thin, rough edges to straighten and deburr before you weld. The good news is that they also sell used on Ebay for about $50 if you decide not to keep it. Check with any local heating and cooling supply shop and you will find a pile of them.

You should definately cut off the extra flange on your floor pans. The welds will be more durable in the factory locations, and it will appear original - which I believe is VERY important to you, at least by the questions you have been asking on this board. I would also reconsider purchasing an original pan - at least for the passenger side - where all the fixings are installed for the wiring, fuel lines, and seat brackets. I even went so far as to remove a new Steelcraft pan from my car after I installed it. It actually sagged from the pressure of me sitting in the car, and I'm not THAT fat!
Jeff Schlemmer

Purchasing an original pan is impossible for 3 synchro cars, Jeff. When punching/drilling the holes for plug welding, use a 5/16" hole. You'll get far, far, far better weld penetration. Did you spray the floor surfaces to be welded with weld through primer too? Trim off the flanges; you don't need them, the floor will fit better without them, you won't have lumps under your sill mats or carpet, once everything else (rockers and chassis members) is solid the floor functions to close the chassis member box sections and to keep your butt off the ground so strength is not an issue. Weld the seat rail nuts to fender washers, then weld them to the floor panels. The load on the fastener will be spread out and better do its job that way in the event of an accident, and even w/o accident as far as that goes.
Wade Keene

Bruce,
I am actually in the same situation as you, replacing my floorpans and steelcraft panels. I replace each of the captive nuts by a screw spot welded from below. I plan to remove the lip all around the new panels. I guess they are intended in case of more extensive repair, provided the internal sills would be in poor condition, not your case nor mine. I used the old floorpans as a template to fix correctly all nuts and screws ( take care to the passenger side captive screws holding the wiring .
Good luck .
Renou

' Guess I'm outvoted on retention of the new floor flanges. Perhaps I didn't clarify that I plug-weld these in BOTH the original horizontal positions as well as the new vertical ones. And of course that means even more holes that have to be drilled rather than punched as my punch won't reach any more than about an inch and on flat surfaces only. I grind the welds flush and I dare anybody to observe any lumps under the sill matts or carpets. Granted, the floors a a lot easier to fit if you cut them off and I don't think Steelcraft ever intended that you leave them on - they're just extra material left over from the stamping that they leave on so as to maintain panel rigidity and integrity during shipment. I think it's imparitive to get the two sheets of metal tightly sandwiched together in plug-welding. To that end, I use a lot of drill screws (and sometimes #8 or #10 bolts and nuts) to pull things together into a tight fit. Yeah, more holes to fill afterward, but that works better for me.

And yes, I always use "Weld-Thru" (this is not a commercial, but I often find that if something says "3M" on it, it works; if it doesn't, it doesn't. On surfaces that won't be painted (like inside rear quarter panel repairs), I'll shoot "Weld-Thru" on it after welding too. ' Don't know if this is good advice. It just makes sense to me.

Question: I'm now also using "3M" "Rust Fyter" in crevices (like the crimps in door bottoms, etc). ' Strikes me that this product must be rather like Waxoyl. Anybody know?
Allen Bachelder

Allen

Have to disagree about the flanges. The steelcraft floor pan for the MK1 has three rounded spots on the tunnel flange that correspond exactly to three rounded protrusions on the tunnel. This had to be design and not just extra metal.

With the exception of extra material at the foot part of the pan that I had to trim off, the pan fits nearly perfectly.

This appears to be one of those toss-up issues. Probably will take the flanges off, but then they would also provide a bit of extra support if welded to the tunnel and sill tube.
Bruce Cunha

The "lumps" under the mats or carpets would come from the flange metal, I would assume that someone doing the job right would do a good job of grinding down the welds. The flanges are indeed there to give the option of patching the tunnel or sill with the flange instead of making a separate piece. The front part is also there to give the option of using the extra steel to patch the frontmost footwell area - in the lowermost toeboard area.
Wade Keene

Bruce,

Of course you're right. They wouldn't have stamped the flanges to fit the tunnel if they didn't intend them to be used.

And Wade, of course it behooves us to do neat work. What's the point in "restoring" a car if you introduce new defects in the process. If the flanges fit tightly and if the welds are ground down properly, we're talking about a "lump" the thickness of a 20 gauge piece of sheet metal. If you can see that under a piece of carpet, you have a better eye than I do! A guy who cannot or will not grind welds down smoothly will have lumps all over the floor even if he does trim the flanges off.

By the way, I do not use the flanges to replace or cover up rusted tunnel flanges, rear heal board flanges, etc. I still build them up with correct sheet metal before placing the new floors on - flanges and all. Then I seal the heck out of it - just to make sure moisture doesn't work its way into the joints.

There's no proper way to do sloppy work, and there are relatively few improper ways to do neat work. I don't see any satisfaction in sloppy work, do you? I think leaving the flanges on makes for, at least a slightly neater, stronger job, but that's just my opinion.

Cheers,
Allen
Allen Bachelder

Ok, I did a bit of a compromise. I cut the back flange and the outer as these sills are wider and still perfect.

I left the transmission tunnel flange on and plug wended it. This flang is narrower and there is slight damage to the edge of it up by the gas pedal.

BTW I did note there is an article in the spring 2005 British motoring flyer that Moss puts out. It has some good pictures. I did not cut the floor as they did (sawsall). May be quicker, but cutting the spot welds was not all that difficult and I was able to salvage the old floor as a template for the seat bolt holes.

One question for those that have done this job. The flange on the outer driver side. is stepped. That is it has a wide flange and then another stepped up flange. The second flange is about an 1/8th or so higher. Original pan was attached to both of these, but just how I am not sure.
Bruce Cunha

Bruce,

The stepped flange is the combination of the so-called "castle section" and the flange on the bottom of the inner sill (the box section just inside of the door sill). Looking at the underside of the car, the "castle section" is the fluted piece running pretty much the whole distance from the front wheel well to the rear wheel well. Using drill screws pull your floor panel down tight to the castle section, as well as the inner sill flange on top of it, of course. By then, you will have drilled/punched enough holes to weld it securely to the vertical surface of the inner sill (if you leave the flange on) as well as to the horizontal surface of the inner sill flange and/or the horizontal surface of the castle section. The amount of castle section showing at this point will vary a little from car to car even though it's not supposed to.

Incidentally, despite claims to the contrary, you CAN MIG-weld upside-down. The biggest problem is keeping from getting burned - I have a lot of holes in my work clothes to attest to that. Strangely enough, especially my sox! Having said that, there is no need to weld upside-down when replacing floor panels, but I have had to do it in sill replacement - like if I forget to weld something while it's easy to reach.

Cheers,
Allen
Allen

' Forgot to mention... this "step" you mention should not be as high at 1/8", does the "upper step" not meet the lower one? or, do you suppose these floors have already been replaced once and a "flange" of the original floor material was left on top of the inner sill flange, building it up to 1/8" thickness? That seems very unlikely, but I'm trying to imagine why it should be any thicker than one sheet of 20 gauge metal.

Allen
Allen

This thread was discussed between 27/05/2005 and 31/05/2005

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