Installing Subframe Connectors in a Classic Mustang

When I first took my Mustang apart, I was shocked to see how the force from the engine was transmitted to the rest of the frame. The front frame rails run into the floor and stop. The outer frame rails that go to the rear of the car are not directly connected to those rails. I understand that the roof carries a lot of the burden of force going back, but something just didn’t seem right.

When the car was driving, the fenders flapped around like crazy. I wanted to make sure this car was properly stiffened. The first step was to add subframe connectors. This would tie the front and rear frames and stiffen the frame. So, I bought a set

…and then immediately took them back. I wasn’t convinced that 2×2 box tubing would do any good when attached with thin-gauge sheetmetal. I had read an article that described putting subframe connectors through the floor and directly into the front and rear frames.

On the top, the original subframe has no connections between the front and rear. The bottom shows the new connections.

The upper left part of the above image shows the basic form of the lower part of the Mustang frame. The lower right side shows the frame with the addition of subframe connectors and torque boxes (described in another area).

To find the dimensions, I measured out where the subframe connectors would need to go. The front of the tube was to go into the end of front frame rail. It would then go through the floor, through the rear torque box and up to the rear frame rail. The image below shows the dimensions as I measured them. I started with two 4′ sections of 2″x2″ box tube with 1/8″ wall. The 3 1/8″ width and 46″ overall length are the important dimensions. Otherwise, you just have to make sure that there’s enough room on either end to slide into the frame.

These are the dimensions of the subframe connector, made from 1/8

I used those dimensions as basic guidelines, but since I couldn’t hold very tight tolerances with the tools in my garage, I decided to measure each cut as I went. I started by marking out and cutting a v-shape out of the 6″ side with my pneumatic cut-off wheel so that it would make a 10 degree angle and welded it together with the MIG. Since the 3 1/8″ dimension is critical, I measured and marked the tube at the right spot after making the weld (since it wasn’t exactly 10 degrees). Next, I marked and cut out the ‘V’, and welded it together, making sure the ends were parallel. I finished the piece off with some weld-thru primer.

Using the subframe connector as a template, I marked along the bottom of the floor where the piece would go. I also marked a 2″ square on the rear torque box where the box tube was going. Then, I used the cut-off wheel to cut the floor down the middle of the mark I made (see photo).

The rear floor pan with the cut for the connectors

Then, using a paint stripping wheel on my drill, I cleaned the area thoroughly of any paint and rust. Next, I bent the floor using pliers to fit the piece and create flanges to weld to. As shown in the photo below, I bent the flanges up behind the seat platform and down under the platform. I used a small piece of matching box tubing to make sure the hole was the right width.

The rear floor pan in bent to accept the subframe connectors

The next cut was made on the front frame rail so that the connector could be lifted in from the bottom (see below). While your looking at the picture, note the connector on the far side already welded in place. The final step before installing the connectors is to punch holes in the flanges for the welds.

This is the front subframe under the car cut to accept the subframe connector

Here’s where the fun starts. In order to get the connector in, you need to start under the car at the rear, slide the end through the floor and into the hole in the rear torque box. Due to the bend in the piece, you’ll need a big hammer to do this. In retrospect, I probably could have helped myself out a lot by putting the diagonal section closer to the front frame, so that it was easier to get into the floor. I put it towards the back because I thought it would be easier to cut the odd shape without the seat platform in the way. Live and learn. Once the end was banged into the hole, I used a floor jack to lift the new piece into place. All the nice work I did to bend the flanges got bent all out of shape during this process, so I used clamps, pliers, and a hammer to get it all lined back up. Once that was done, I spot-welded the ends and various points in between (the first two photos below). I re-checked everything, then finished the welding. Check out the last picture below to see what it looked like.

These are the spot welds from the floor to the subframe connector

This is what the welds look like in the rear

These are the subframe connectors in the floor

To finish the project, I ground the welds smooth, put body seam sealer in the seams and sprayed undercoating on all the areas.

The subframe connectors in place and a protective coating sprayed on