Installing Front Torque Boxes Into a Classic Mustang

Unibody pony cars have had a bad rap for years about saggy, weak frames. I can’t exactly argue that point. If you take a look at the frame and sheetmetal in the front of Mustangs, it actually seems a little scary. The front frame rails go to the floor and stop. That means that the load from the engine is passed from the frame rails to the rest of the car through three areas:

• Through the floor to the outer frame rails (rocker panels)
• Through the inner fenders to the firewall (my last Mustang had a small buckle in one of these panels)
• Through the shock towers to the firewall IF you have a shock tower brace.

None of these areas are very thick, and I could imagine them deflecting and bending pretty easily, leading to a sloppy frame. Not to knock the designers (I’m sure they knew much more than me about this stuff), but I know I’d sleep better if I did something about it. So, here was the plan:

I installed in-floor subframe connectors to tie the front and rear frame rails (described in another section) and then got to work on putting in torque boxes to tie the front frame rails to the outer frame rails. I figured that if there was ever a time to put torque boxes in, this was it. I’m sure most of you know that this was the standard routine for convertibles along with some other goodies to reinforce the frame. Consequently, torque boxes are easy to find in the aftermarket. I bought mine from JC Whitney for $60 each before shipping. They seemed to be good quality, of a sturdy thickness and they fit well.

The new torque box from JC Whitney

Getting Prep’d

The first step was to figure out how these things fit in. I decided that removing the floor would be more work than it was worth, so the boxes would need to come in from the bottom. To get this to work, I had to make some slight modifications:

	Shave off the lip from the outside of the front frame rails where the boxes would go.
	Shave some material (about 1/8″ to 3/16″) from the torque box flanges that mate up against the floor and front frame rail. This flange is SUPPOSED to be sandwiched between the floor and frame, but I couldn’t figure out a way to do this without removing the floor, so it just had to be stuck underneath.
	Cut a slot in the floor where it’s attached to the outer frame. This allows the outer flange from the box to pass through. The floor is re-welded down to the box later. Also, there is a small angle bracket inside the interior where that last cut was made. The spot welds need to be drilled and the bracket removed.

The next step was to clean the entire area really well. If there is any seam sealant or road grime where the boxes are going to be, it all needs to be wire-wheeled away to allow them to slide up into their spots. In the areas to be welded, I used a stripping wheel on my drill to get it down to the metal, then acetone to make sure it was really clean, then weld-thru primer for rust prevention. Don’t forget the area inside the interior where that cut was made along the floor.

Next, I drilled holes in the box flanges for welding. I used a Unibit indexed sheetmetal bit and would recommend you do the same (you can get them from Eastwood or Harbor Freight Tools). Holes were placed on every flange except where it was attaching to the floor. In those spots, I drilled holes in the floor instead, since it’s thinner and easier to weld through to the box.

For added precaution, I applied weld-thru primer to all the welding surfaces of the box and undercoating to the inside of the box (careful to mask off the welding surfaces).

Fitting the Box

One of the torque boxes dry fit between the rails

Here’s where it gets fun. To fit the pieces in, I used a floor jack to force them in from the bottom. I used a hammer to make adjustments as it slowly fell into place. Then, I checked and re-checked all my surfaces to make sure that (a) they all mated up tight and (b) the welding surfaces were clean. I actually had to back out the first box I put in to clean the interior surface I reminded you to clean in the last section.

The rest is gravy. I welded up the dots, ground them smooth, applied seam sealant, and undercoated the area to dampen the noise, keep out the moisture, and make it look pretty.

Torque box welded and undercoated