Mustang Rear Torque Box Repair

Purpose:
1) Repair rusted torque box covers.
2) Repair the bend in the torque box floor from years of using it as a jacking point.
3) Improve overall stiffness and strength.

Before

After
After removing the rear floor pan I found the lip of my mustang rear torque box rusted. At this point I wasn't very enthusiastic about tearing the rear torque box apart and I had no real plan as to what I was going to do about the rust.
The best thing I could think of initially was to just take a peek inside the rear torque box and see if there was extensive rust inside or not. As you can see it's pretty clean inside because of the galvanized coating. The only rust is where the spot welds are and it's just slight surface rust.
While deciding on what I was going to do I realized the floor of the rear torque box was bent up wards from years of using it as a mounting point for jacking the car up. If you look at your rear torque box on your mustang, yours probably has the same problem. This problem and some more brainstorming caused me to go ahead and cut open the torque box completely.
I cut it open leaving the lip with the spot welds on as this was easier then cutting out the spot welds right now. This turned out to be a good move because it helped in measuring the flat bar pieces I welded in.
Fixing the bent floor of the torque box wasn't that big of a problem. I propped up the front lip with a floor jack and whacked the middle with a big hammer. Came out well. The problem now is how to prevent the same thing from happening again ?
I made a mock up of a support frame inside the rear torque box. Here the floor will be stifferend and strengthened so that if it is ever used as a jacking point it won't bend inwards again. On the opposite side of the back wall of the torque box is the attaching point for the leaf springs. This is a very critical area, especially if you plan on being rough with the vehicle and doing 360s later!
I cut the flat bar pieces (I think they are 1/4 inch thick) and put then into place. The fit looks good, and all I need to do now is grind off the galvinizing where I will weld.

I welded the flat bar to the torque box using a full bead. It wasn't necessary, could have just layed down a few spaced out welds but it was a good place for me to practice my welding and it can't hurt it. So if you have plenty of gas and welding wire, you can go all out. In the third pic you can see how the flatbar and the subframe connector come together on opposite sides of the torque box wall.


The reproduction rear torque box cover isn't a perfect fit but it's good enough! Just a little bit of trimming and clamping and it will fit well. But first we have to drill the spot weld holes.


I drilled 24 spot weld holes in the torque box cover plate. I forget what diameter drill bit but I think it was 5/16ths.
After prepping the understide of the cover plate, I put pieces of foam (ear plugs also work great) into the drilled holes to keep the sides of the holes from being painted. Although I'll be using weld-thru primer to coat the lip I still don't want the primer directly painting any area I will weld. Weld thru primer is good, but not that good. I use it to protect the areas directly around where the weld will be put.

Here is some final prepping before I weld the rear torque box cover plate on. I used some 3M seam sealer on the bare welds then sprayed a coat of undercoating on top. I left the lip bare metal and only sprayed weld through primer in those areas.


Clamped and ready to be welded. You will notice in the second and third pic that I cleaned out the weld through primer in the drilled holes. As I said before, weld through primer has it's uses but if you can clean the area and weld to bare metal then that is a better choice. This way I think is the best way to protect against future corrosion.


Since the prep work was done, the welding was very easy. I'm very happy with how this project turned out. Hope it helps you in replacing your rear torque box cover plates!