MGB Floor Replacement
Removing the floors on an MGB
By Jonathan Lipkin, with help from: Jack Austin, Steve Lyle, Paul Gaylo, Peter Plouf, and others.
The floors on MGBs are made from sheet metal and secured to the castle rails and crossmembers by a series of spot welds. These welds run along the castle rails, a flange about 1” wide that runs along the transmission tunnel, the rear of the car. Along the transmission tunnel, the floor sits on the transmission tunnel flange. The floor is also welded to the cross members. By carefully drilling out the spot welds, you can remove the floors where they are mounted to the car, allowing you to properly weld in new floors.
For the angle grinder:
Cut off wheel (optional, you can also use a reciprocating
saw to cut)
5/16 drill bit - you’ll need more than one
1/8 drill bit (optional)
Center punch (optional)
3/8” spot weld cutter
Box and postage for the spot weld cutter to return it
when it stops working
Tongue and groove pliers (sometimes called channel
Goggles or face shield
Angle grinders generate a lot of torque and make quite a racket. When using a wire wheel, they can catch in certain situations and launch themselves across the room. Let go. Also, the bristles have a tendency to come loose. After the first time I had to remove one from my cheek I began to wear full-face protection. They will also end up all over the floor and then work their way into your skin as you crawl around under the car. As you grind and wire wheel away paint, it will produce small paint chips (which may contain lead) and smoke that you probably don’t want to inhale.
To start, you’ll need to remove the carpet, the seats and all seat mounting hardware. Remove the top. It’s a good idea to remove the steering wheel. As you remove the carpet, be careful of the covering of the transmission tunnel as it may be made of asbestos. Also, you might want to cover things in your garage, as all the grinding and wire wheeling you’ll do is going to make a holy mess. Some people recommend removing the fuel and brake lines which run under the passenger side of the car, though if you’re careful you can leave them in place. Just be very very careful when you are cutting around them. Cutoff wheel + fuel line = bad day.
At the least you’ll need to remove the brackets that hold them in place so you can move them out of the way as you’re cutting through the floors. They will also get in the way when you are painting the new floors from underneath. Before you proceed, make sure to mark where the seat mounting hardware goes, along with the underbody harness support studs. These are not installed on some replacement panels and are easier to install out of the car. If you get the Heritage panels, these are pre-installed so there is no need to do so.
Floors completely removed. The next time I do this, I’ll be sure to remove the steering wheel, console and top before I start. Some people suggest that you support the crossmembers with a jack or 2x4 during this process. I jacked the car up and put it on stands because I find it easier to work on the car when it’s higher in the air. There’s a lot of bending involved in this job
Cut away the floors to about an inch or so from the castle rails (a structural element of the sill structure that runs along the sills), the cross members. Castle rails protrude from the sills about an inch, producing a small lip that the floor sits on. I found the best way to mark their position, and the position of the cross members, was to crawl underneath the car and drill holes from the bottom as a guide. Then, cut from the top using a cutoff wheel or reciprocating saw. Remember to be very very careful not to cut the brake/fuel lines, muffler and/or cross members. Save the pieces of the old floor so you can use them to practice your welding.
If you look from the sides, you should now be able to see the floor, which is 20 gauge steel, and the castle rails, which is a slightly heavier gauge. At the factory, the floors were secured to the castle rails by a spot welder. Assuming you have factory floors, removing the spot welds will allow you to remove the remnants of the floor. If the previous owner installed new floors, they may be held in place in any number of ways - rivets, glue, chewing gum. You’ll need to entirely remove the old floors in order to properly secure the new ones. Some people will leave the remnants of the old floors in place and weld new ones on top. This is not a good idea, as there is almost certainly a layer of rust between the floors and castle rails.
In order to remove the spot welds that hold the remnants of the floor to the castle rails, you’ll want to remove the paint above the spot welds to make them easier to see. A wire wheel is a good way to do this. Move along the castle rails all the way around the floor pans, then along the cross members. Spot welds are located about 1” apart and can be identified as a small indentation in the sheet metal where it is joined to the castle rail. Once you’ve found what you think is a spot weld, drill with your 5/16” drill bit in the center of the indentation. Some people drill a 1/8” hole as a pilot, or use a center punch. The goal is to cut only through
the top layer (the floor) and not the castle rail. However, if you are comfortable welding upside down, you can drill all the way through the castle rail and then do your puddle welds from underneath. Leaving a small hole from the 1/8” drill is ok, as it will get filled in by your puddle welds later. Stop occasionally and examine the hole as you’re drilling. You should be able to see when you’ve cut through the floor before you get through the rail. Be careful, as it’s very easy to apply too much pressure and cut all the way through. If you do, it’s not a big deal, but it will be a bit easier to weld in the floors if you haven’t drilled through the castle rails. Use your spot weld cutter, which looks like a center point drill bit, for the first ten welds - it’s much easier to use than a plain drill bit. After the spot weld cutter becomes too dull to use (for me this was after five welds/thirty seconds), put it in the box and mail it back for a refund. Mine cost about $25 from Eastwood, and there has been suggestions that if I had used a different cutter it might have worked better.
Some people have had better luck with spot weld cutters. You can tell that you’ve cut the weld by putting the pry bar between the the floor and castle rail. The floor should pop off. I found various ways of separating the two, but the best was to move along the castle rail with the pry bar between the castle rail and the remnants of the floor until I met resistance - this was the next weld. Then I’d drill it out and take the tongue and groove pliers and pull on the floor. This would clearly show me where the spots welds were and if I had fully drilled them out.
Another method is to simply use an angle grinder to remove the spot welds. Steve Lyle recommends doing this as it is less destructive to the underlying flange. Paul Gaylo recommends using a cold chisel. Some people simply go at it without drilling. In this case, you’ll need to be extra careful as it’s a very aggressive tool. Keep the point sharp. Paul drills the welds, then uses the chisel at just the right angle and is able to pop the weld the rest of the way off.
After you’ve gotten the remnants of the floor out, you’ll need to clean up the castle rails so that you can properly weld in the new floors. Take a wire wheel to remove any rust you see. Then, use a grinding wheel to remove any high spots or bits of the floor that are still attached to the castle rail. You want to leave a flat, clean clean clean surface to weld the floors to. It is not a bad idea to paint the inside of the cross members with protective paint. I used POR-15. Some people also put in a bit of Waxoyl. The next step, of course, is to get some panels and weld them in.