MGB Sill Repair
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The sills on an MGB, particularly the Roadster, are an important part of the structural strength and integrity of the car and should always be in good condition. Any serious rust can weaken the body and cause the car to sag in the middle. This will be evident, especially on a Roadster, by uneven door gaps. The sill repair usually consists of these panels:
  • The outer sill panel, part of which is visible under the door.
  • The castle section
  • The flat inner sill panel, which is sandwiched between the outer sill and side members
  • Sill end filler.
  • Front and rear wing lower repair panels - as appropriate.
It should be noted that these panels are 'handed' for either left or right side of the car. In addition to these repair panels there is a jacking point reinforcer, which fits inside the sill over the jacking point and should always be replaced since it cannot be accessed without dismantling the sills. The jacking point itself, which is exposed to the elements, could also need replacement. Before fitting, it is a good opportunity to paint the inside surfaces of the new panels for protection. Any areas that are to be welded should be left free from paint unless weld-through primer is used.

The side member is generally in good condition often only needing minor repair at the front where it joins the bulkhead. Care must be taken when carrying out repairs in the sill area to maintain the structural integrity of the body and it goes without saying that welding work should be of high quality.

As always, fire and safety precautions should be observed to ensure that work is carried out safely without risk of loss or injury. The car can be raised to a convenient height to work on the sills and should be supported on suitable axle stands. If the rear axle is still in position, place a suitable block of timber between the rubber bump-stop and the axle. Then rest the axle on a stand. An axle stand can also be placed under the front cross-member. Place a length of timber across the car under the central cross-member and rest it on a screw jack to support the centre. If a hydraulic jack is to be used you should be confidant that it does not leak or loose pressure over a period of time. When the supports are in place the car should be level and should not be able to flex on the suspension because any movement could affect the alignment of the central support.

Before starting to dismantle the car take note of where the sills fit and the alignments between the front wing/sill, the rear wing/sill and the door/sill. The sills stretch the length of the car from just in front of the rear wheel to the bulkhead just behind the front wheel. To carry out these repairs properly the full length of the sill must be exposed. Either the front wing can be removed or if not, the lower part can be cut away to expose the front section of the sill. Similarly the lower part of the rear wing can also be cut away to allow access to the rear section of the sill. Fortunately there are repair panels available to replace these sections.

Carry out repairs to one side of the car before starting the other. This will help to maintain structural integrity and give you a useful reference point when building up the other side. To start, the remains of the old panels must be removed. First remove the front wing or cut out the lower section of the front and rear wing using the repair panels as a guide. Cut off only enough material to allow access for the repair. Final trimming can be done when the new panel is to be fitted. Once this has been done the extent of the sill will become apparent. Before removing the rusted sill sections ensure that the centre of the car is properly supported. The next step is to remove the remnants of the old sills and the jacking point strengthener, which were spot-welded into place by the manufacturer. If necessary the jacking point can also be removed at this time. You are now left with only the side member exposed and in place.

Make any necessary repairs to the side member and dress the flanges ready for fitting the new panels. The castle section is the first replacement panel to be fitted. Before offering up the new panel, make up three templates using of-cuts of sheet metal. Each template should measure 145mm by 45mm and have four 3mm  holes, one in each corner, so that they can be firmly attached to the upper flange of the side member and the outer flange of the new castle section by Cleco fasteners. At the same time drill more 3mm holes in the inner edge of the castle section on a line where it will butt up to the side member. Offer up the castle section from underneath and align the inner edge against the lower flange of the side member ensuring that it sits comfortably with the shape of the side member. Drill further 3mm holes in the side member flange so Cleco fasteners can be put into place to secure the castle section to the side member. The tongue at the front of the castle section will be dressed upwards and welded to the bulkhead later when the rail has been securely welded into place. Next the three templates should be put into place, one at the front, one in the centre and one at the rear along the length of the sill. Fix them in place with Cleco fasteners as shown in the picture. They should be arranged vertically so that the gap between the top of the side member flange and the bottom of the outer castle section flange is maintained at a constant 145mm along the length of the sill.

Once you are happy with the fit of the new castle section, weld along the seam between side member and castle section inside the sill. A continuous weld is not necessary but weld approximately 25mm lengths with 25mm gaps. When this is done dress up the tongue at the front and weld it to the bulkhead. After the castle section has been welded into place remove the three templates and ensure that the 145mm gap has been maintained.

The next step is to fit the jacking point reinforcer. This fits over the jacking point on the castle section inside the sill. Welding in should be straightforward but the reinforcer may need to be trimmed to fit into place.

The inner sill will initially be trial fitted using Cleco fasteners so drill 3mm holes around the edge at 50mm intervals for later alignment with the side member and newly fitted castle section. Offer up the inner sill and initially clamp it into place. Once correctly aligned, drill corresponding 3mm holes and fit Cleco fasteners to secure the sill into place. This is not a difficult panel to fit. Once you are happy with the fit punch plug-welding holes around the edge at 15mm intervals at points where the panel meets the castle section and side member flanges. Refit the panel and carefully plug-weld around the edge to secure removing Cleco fasteners as you progress. Following this the plug-welds should be dressed flat ready for the outer sill to be fitted.

Since parts of the outer sill is visible care should be taken to achieve proper alignment with corresponding wings. At this point the door should be fitted and properly aligned in its aperture. The outer sill can then be aligned with the door taking care to allow plenty of clearance between the bottom of the door and top of the sill. As previously, drill 3mm holes at intervals around the sill flange for Cleco fasteners. Offer the sill up to the newly fitted inner sill and align the front stepped recess with the front edge of the door. When the front wing is fitted later, it should fit over the front part of the sill and align neatly with the door. The gap between sill and front wing should be the same as that of the door to wing. The step towards the rear will be aligned later when the rear wing repair panel is fitted. The top flange of the sill should be aligned first with the top edge of the inner sill. Once the top flange has been secured the bottom flange can be aligned. The bottom flange may need to be flexed upwards to align properly especially at the front where the front wing fits over. A scissor or bottle jack and block of timber together with lots of clamps will be useful. Once the panel has been aligned, remove it and punch plug-welding holes at 15mm intervals around the flanges. Refit the sill and plug weld in place.

The final piece to be fitted is the sill end filler. This piece fills the end of the outer sill at the rear and should be a tight fit. Plug-weld it into place and later, ensure that plenty of seam sealer is applied to prevent water and dirt being driven into the sill cavity by the road wheel. When these sill panels have been fitted it only remains for the front wing and to rear wing repair panels to be put in place.

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