The number of times 1100's turn up with the dreaded black sill disease is quite amazing. This look is usually down to one of two things. Genuine 1100/1300 sills have been "protected" from the dealers like this, or a cover sill has been crudely welded over the top of the existing panel. The result on the outside is the black underseal.

Sills are an important part of the bodywork on the 1100 as they provide strength to the shell, seat belt anchoring points, and also jacking points.

The sills are made up in four parts. Shown in the diagram below.

Usually if the outer sills are rotten the bottom of the membrane will also be affected, together with the closing panel, and the inner sill where the latter bends to form the floor pan. This is not always the case. Some cases the car has been well looked after and people have used a waxed based rust protection product down the sills and membranes. But this doesn’t always stop the outer sill going rotten. It is possible that your sill to floor closing panel is ok, but your outer sill is rotten, and visa versa.

You can get cover sills which, as the name suggests, cover all of this area. But these, if fitted can cause longer-term problems of floor rot and generally decay the area covered. This type of sill also does not pick-up the lower part of the membrane and so it is not as strong as the original design.

Other things which may be classed as a problem are that the outer sill on four-door models finishes half-way across the rear door aperture and any decay past this point will have to be dealt will separately, either using an extended sill, or a body section which also includes part of the rear wheel arch.

Two door models only go as far as the ‘B’ post. It is advisable to have a strip of 18swg mild steel some 6 ft long 3 ins wide to repair the bottom of the membrane if necessary. It is not necessary to remove this part as decay is easily ground away and new steel welded in position following the line of the original section.

If the inner sill is corroded where it turns to form the floor the instead of trying to replace the whole of this part, angle can be welded into the corner, but remember the seat belt anchorage is attached to this panel, so pay particular notice to welding in this area.

Tackling the task

Remember that safety is a must when working on anything. Particular on a vehicle which will be suspended in the air. Support the car on axle stands underneath front and rear subframes at a height that enables welding to the floor pan, or jack up the car, and block the wheels at both side, ensuring that the car is safe. Remove the doors on the side to be dealt with.

With a grinder and chisel remove the outer sill.. The new sills come complete with the step section, however the step section is rarely rotten, so if you wanted to you could remove this step section as shown in the diagram below. During the re build of my own car, we left the step section on the new sills and removed the existing. We found it slightly easier doing it this way.

You will now see the inner sill membrane it is important, at this stage, to note the amount of work required in this area. If there is no corrosion, either to the membrane or closing panel, it might be advisable to clean off any surface rust and use Red Zinc Oxide or a weld through primer and paint the whole of the area. Then the new sill can be welded in place, trimmed and painted, this usually isn't the case, and more prep work needs to carried out.

The sill to floor closing panel should now be removed by grinding away from the lower part of the membrane, and the floor as the shown below.

From this point its now a matter of reconstruction. Before welding its a good idea to line panels up using mole grips and jacks. Position the closing panel to the floor and membrane, and weld in position. A small MIG welder is sufficient for the job, and make sure that there are ¼ins holes drilled along the edges of the new panels where you will them to the bodywork.

Having successfully fitted the closing panel and repaired the membrane, then all that remains is to fit the outer sill. You might have difficulty in tucking the front edge of the panel behind the lower part of the front wing. If so cut away metal from the sill, but remember to weld at this point. The joint at the other end of the sill is a straightforward butt. You might have difficulty welding the two parts together.

Having completed welding and trimming with a grinder, the joints with the step and rear panel should be finished with body filler, and when desired standard have been accomplished, painting can take place

Remember the wiring harness to the rear end of the car passes through the offside sill. Do not attempt to carry out any welding or cutting with this ‘in situ’; it is easily removed and replaced with a ‘Pull wire’.

This article originally appeared in Idle Chatter August/September 1990. I have however used the original as a basis, and where appropriate made alterations as other methods have been found of doing the job.