FLOOR PAN REPLACEMENT
MGB Roadsters can be less than waterproof and as a consequence can suffer from rust in the floor pan area where water has penetrated. If the rust is severe, especially around the rear spring hanger area and the foot-wells then often replacing the floor pan is the best option. New panels are readily available from specialists. Floor pan panels are handed i.e. there is a right side and a left side. Also the three-synchro car has a different shape floor pan from the four-synchro car. However, replacement is basically the same in all cases. The heel board reinforcing panel above the spring hanger should be replaced at the same time.
Before dismantling begins the normal safety precautions should be taken. The batteries should be disconnected and the fuel line/tank should be protected from fire hazard. If the car is to be raised care should be taken to ensure it is properly supported to prevent injury.
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Removal of the old floor pan panel should be straightforward. But first, the wiring loom, battery cable, fuel line and brake line are secured beneath the floor on the right side. These should be detached and moved out of harms way. Removal of the old panel is best tackled by cutting out the three sections between the under-floor cross member and longitudinal member as shown in the picture. When cutting out the rear section do not cut into the strengthening panel underneath the floor that bridges the rear of the central tunnel. It then remains to drill out all the spot-welds and remove the remains of the old metal. The flanges left behind after the remains have been removed will be used again for fitting the new floor pan so they should be left in tact. When the heel-board reinforcing plate is removed the car will be weakened at this point and so should be supported until the new plate is welded into place.
At this point the condition of the cross/longitudinal members can be ascertained. Some localised repair may be necessary. Replacement of the jacking point is also possible but this is generally done with sill replacement. It is also possible to paint the inside of these members before the new floor pan is fitted to help prolong their life.
Before fitting the new panels you should ensure that the captive nuts for attaching the seats and captive bolts for attaching the under-floor ancillaries are in place. Some replacement panels are not fitted with these essential items. For the seat fixings, ¼ inch UNF nuts should be welded in place and for the ancillaries ¼ UNF set screws, 5/8th inch long can be used.
Next comes fitting the new panels. The heel-board reinforcing plate should first be welded into place over the spring hanger. This is safety critical because it contributes to the strength and stability of the spring hanger. The new floor pan panel can then be welded into place. This will take a number of trial fittings before it is ready for welding. Once satisfied with the fit holes should be punched around the outer flanges ready for plug welding. Also, the new floor pan will be welded to the cross/longitudinal members so carefully mark on the underside of the floor pan where the members will fit and then drill holes for plug welding.
Fit the floor pan into place and commence welding. Plug-weld at intervals around the floor pan and ensure that the flanges being welded are pressed firmly together to form a good join. Take particular care around the flange where the new floor pan joins the heel-board reinforcing plate. Seam welding is advisable at this point to ensure a strong weld around the spring hanger. Welding is a lengthy process and should not be rushed to avoid excessive heat build-up, which leads to distortion. Be aware that welding can be hazardous and the necessary fire precautions should be taken.
Once the new floor pan is welded in place the seams, both inside and underneath should be sealed with seam sealant to prevent water ingress allowing the rust to starting all over again. Following this depending on the finish required, the under-side should be painted and/or undersealed and the ancillaries put back into place. Inside, new soundproofing should be attached to floor. Self-adhesive soundproofing shaped for the four-synchro gearbox floor pan is available but not for the three-synchro car. However, the four-synchro soundproofing can be cut to fit the three-synchro car. The soundproofing material fits better if it is first heated with a hot-air gun. This makes it more pliable and also helps the adhesive. The inside surface of the floor pan can then be painted as desired and the carpets and seat can be replaced to complete the job.