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MG MGB Technical - Floor Pan Replacement

My project continues unabated and, having found a few holes in the floor panels when the seats came out to replace the handbrake, I did the sensible thing, bought a welder, and have a couple of weeks off to learn how to use it.

A few online sites offer some guidance but are rather brief. Can I ask for any practical experience and tips please from those who have done this before. First and foremost, will an angle grinder and cutting disc be sufficient to remove the old floors, or do I need something altogether more specialised - please don't mention a gas axe...
A Riddett

I've done it a couple of times now. An angle grinder and cutting discs are good. Sanding and grinding discs are good too. I also used a metal chisel and a joggler tool with a metal hole punch on the head to make holes around the edge of the new floor pans to plug weld through (although you need to drill the middle ones still).

I found it easiest to cut out three large sections of the old floors around the cross members. You can measure and draw guide lines on the top of the floor to know where to cut. That then left me a thin strip around the edges and over the cross members to remove.

Then I used a sanding disc on the angle grinder to sand the edges of the strip to find the spot weld points. I then sanded them until they were paper thin and then used the hammer and cold chisel to pop off the strip. If you sand them all thin enough you can grab one end of the strip with pliers or vice grips and often pull a lot of it up all at once.

Then sand flat the remaining bits of spot weld left on the body. I used the punch then to make the holes in the edges of the new pans. To make the holes in the middle of the floors where it needs to weld to the crossmembers I dropped the floor in then from underneath traced around the edges with a marker pen. Then you pop them back out again and you know where to drill the holes.

I plug welded the floors in after making sure the nuts for the front seat mounting points are there as they are inside the horizontal cross member when the floor is in and are inaccessible (I also added a bit of weld to them so I know they won't ever pop off). Make sure the floor is pushed down hard when you plug weld. The welding can make it distort a little and lift slightly. I then seam sealed under neath the car then, once dry, literally poured rust preventer into the top of the floors and let it all seep in. After cleaning the excess I seam sealed the top.

I vaguely wrote about it back in 2005 here:

Simon Jansen

A spot-weld drill is also useful.
Dave O'Neill2

You must remove all trace of the old floor metal, and that means grinding off the edges, and/or drilling out the spot welds at the edges and over the underfloor structural pieces, and on my car there were 107 floor spot welds on each half of the car - I counted them. I used about a dozen spot weld drills (they go blunt) and an air-powered chisel. It is a horrible job, especially welding down in the footwells when you have to hold your breath, weld a bit, and come up for air. You will probably find other nasties once you get in among it, especially the triangular brace where the rear spring hanger is attached.
Mike Howlett

Cheers Simon, just read your recently resurrected thread on stripping out the engine bay, and have basically decided to do likewise. I'm not going to get the car back on the road this year - I have to pay tax on my 73 Roadster - and since I'm away most of next summer I might as well turn it round, unbolt things and have a good old tidy up.

Cheers everyone else, added to shopping list. I was surprised to learn that the floors are only fixed in by spot welds not a continuous seam. At least it gives me a direction to practice in!
A Riddett

As Mike says, make sure the welding points are VERY clean or splatter and poor welds will result.
Don't be frightened to turn up the power on the MIG especially on the chassis and castle rail sections to get good penetration and strength in the welds. I also use a weld through primer on all the hidden faces.

best of luck

M McAndrew

There are no seam welds in the MGB body. It is all held together by spot welds.
Mike Howlett

Spot weld cutter is what I used. Others have used air chisels. The cutter works well, but remember, there are a ton of spot welds (I seem to remember somewhere in the 80's) per floor board.

Once I cut the welds out, I used a grinding stone on a hand grinder to grind down the weld plug left by the cutter. I took mine out in one piece so I could use it as a pattern to where the old holes were.

After that, it was a lot of welding to fill in all the holes. When done. While not quite the way it left the factory, it was in very well.

I also recommend using weldable primer on all bare metal and seam seal around any edges.
Bruce Cunha

Simon,how are things going with the car restoration?Rich O
rich osterhout

Hi Rich, I guess you mean the Austin 7? Getting there slowly. I have to spread out the costs. I almost have the frame tacked together ready to be removed and made all nice. Then I will also get the chassis to the rolling stage. I have restored all the front suspension/steering ready to go on and am working on the rear axle now.

Got a bit distracted putting an OD box in my MG finally a month or so ago. Is so much nicer to drive now!

Simon Jansen

This thread was discussed between 21/08/2012 and 30/09/2012

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