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The MGB boot floor can suffer from rust both on the inside due to water ingress and on the outside where it is exposed to the elements. Water and dampness can cause havoc if it penetrate between the fuel tank and boot floor. Often the best remedy is to replace the floor as a whole.

The first step is to raise the rear of the car to a convenient working height. As always the car should be properly secured when working underneath. The process involves welding and it is a wise precaution to have a suitable fire appliance on hand.

Remove the fuel filler inlet and fuel tank, which is mounted directly beneath the boot floor. Ensure that the short length of wiring harness to the sender unit is placed out of harms way. This would be a good time to replace the tank because if the boot floor has rusted then the top of the tank will almost certainly have suffered from the bug. Store the fuel tank well away from the work area. The fuel line from tank to pump should also be removed and replace if necessary. Part of the wiring harness inside the boot that supplies the lights is vulnerable and should also be moved to a safe place. The rear section of the exhaust pipe is suspended from a bracket mounted on the underside of the boot floor and so should be removed. At this point it is also prudent to remove the rear bumper to facilitate access. Finally, the batteries and fuel pump are mounted close to the work area and should be removed to prevent any risk of damage or fire.

The next step is to remove the old boot floor. Before you do this you should examine the new floor to get an idea of its extent and how much old metal should be removed. The new panel should have a rear exhaust-mounting bracket attached to the underside and a spare wheel fixing mounting on the top. This panel is readily available from good specialist suppliers. Using an angle grinder cut out the old panel making cuts approximately 50mm inside the rim of the floor where it has been spot-welded into place. Once the bulk of the panel has been removed drill out the old spot welds and remove the remnants of old metal. An angle grinder or nibbler would be most suitable for cutting to avoid any distortion of the remaining metal and a 6 or 8mm spot weld drill bit should be used for drilling out the spot welds. Be especially careful when drilling out the spot welds along the rear valance since this is an exterior panel. Following this, dress the flange around the boot area with a hammer and dolly ready for welding in the new panel.

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MGB Roadster boot floor repairsAs always, great care must be taken to obtain a good fit for the new panel. Pattern panels never quite fit first time and some cutting and shaping will be necessary. Offer up the new boot floor into place and make minor adjustments until you are satisfied with the fit. The floor will need to be taken in and out several times before this happens and so Cleco fasteners spaced around the rim will make the fitting and removal less frustrating. Once a satisfactory fit has been obtained, remove the panel and punch holes around the flange at 15mm intervals ready for plug welding. Refit the panel and plug-weld it into place. The exhaust mounting bracket underneath the floor will also need to be welded to the rear valance. When this is complete dress the welds as necessary with an angle grinder ready for repainting.

Underneath the floor all seams should be sealed with seam sealer before painting and undersealing as desired. During welding the outside finish of the rear panel may be damaged by heat and some repainting will be required in this area. Finally, to complete the job, refit the ancillaries previously removed and road-test the car.

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