I recently spent a frustrating weekend trying to correct a problem on a Minor sill and in the process discovered some interesting things about the sill construction.
The car has had a replacement half floor panel installed complete with the inner sill, boxing plate and a new cross member. At the cross member there was a gap of about 10-15mm between the bottom of the boxing plate and the top of the cross member. The restorers who installed the floor were puzzled by this and it had me a bit baffled as well. I had spent about 12 months looking at this (this car is progressing VERY slowly) and finally decided to try to correct it.
I spent several hours dismantling and measuring up another car that is in original condition. It became obvious that at least part of the problem was that contrary to the normal drawing the inner sill wall is not in fact vertical. It appears to slope outward slightly which has the effect of lowering the height of the wall and reducing the gap under the boxing plate. However it was not enough get rid of the gap completely, the remaining gap (about 7mm) I can only put down to a small inaccuracy in the panel construction.
To make matters more complicated the inner sill wall does not have a constant angle. At the front of the rear seat base it appears to be vertical and rolls outward as you come forward along the sill.
To install a full new sill I found that the best way seemed to be to start from the back at the rear seat base and slowly work the angle outwards propping it as necessary. The alignment points are the bottom of the inside side panels at the front and back of the door on a 2-door and the `B’ post on a 4 door. I also used a builders square held against the inner sill wall and compared the height to the floor with the original car. The car needs to be extremely well braced before any of the sills are removed to prevent it sagging.