Whilst we call this a Fire Wall it is in fact a fluid barrier. Its intention is to reduce the possibility of petrol finding its way into the cockpit in the event of an accident. The aluminium sheet should be sealed and riveted in place to ensure a fluid tight seal. The photo shows where the barrier fits.
This part will need trimming and bending to obtain the best fit and sealed in both sloping and flat floor cars. Extra material will need to be removed along the bottom edge to fit to a flat floor car where the floor is higher and where there is a hump for differential clearance.
The barrier needs to be bent at the top to fit into or under the top back board clips. The bottom edge needs to be fitted so that a flange is made. The flange should be made so that the barrier can be riveted to the floor. This will involve fitting over the diff hump on a flat floor and the tunnel on a sloping floor car. (The photo shows a flat floor TR3A). The final shape will be similar to the backboard plus the flange for the bottom fixing.
Firstly the sides need to be bent over too so as to make a flange to rivet in place, or an angle plate is made, riveted to the wheel arch, then the main sheet can be trimmed with a flat edge and riveted to the angle.
The top is the really tricky part. On an unpainted car you could weld a flange to the tonneau rail to fix the sheet to. With a painted car there is no easy way. Usually we rivet the sheet under the 1" wide clips, the upper edge having been bent up to meet the tonneau panel. This can be done 100 different ways, there is no one definitive method.
There will be a gap between the inner quarter panel and the outer quarter panel. It is difficult to seal easily with the outer quarter panel in place. We would suggest putting a piece of 'cut out to size' cardboard in position and pushing fibreglass matting soaked in resin into place up against the cardboard.
To recap then, a flange is needed around the wheelarch up the side of the firewall and around the curve of the wheelarch to meet the side panel. Another flange is made at the bottom of the firewall to meet the floor. these flanges are usually 25mm wide. The top of the firewall does not need a flange but should be forced up to meet the tonneau panel This is not dificult as the fire wall is quite thin aluminium.
The depth from top to bottom of a sloping floor TR2-3A and a flat floor TR3A-B differ by 50mm. The firewall as supplied will therefore be way too long for a sloping floor car and will need trimming.
Once in place the firewall should be sealed all around.