Avanti Torque Box Replacement


The torque boxes (or "hog troughs") in an Avanti are perhaps the best known weak spots in the these cars. They are essentially foot wide sheet metal channels, bonded and riveted to the body floor between the frame and the rocker panels the full length between the wheel wells. They were designed to make sure the fiberglass body would not flex on the frame. After exposure to the elements (and road salt in some parts of the country), they start looking more like swiss cheese than sheet metal.

I purchased R4141 a few months ago with the full understanding that at least the left torque box was perforated. There were also some minor "fixits" needed (tach, power windows, trunk release, and a few others). The rest of the car, however, was in good condition. The paint was very nice "driver quality", the interior needed only carpet and repairs to the driver's seat, a bunch of mechanical work had recently been done to the car including all new brakes, new suspension and steering components, new springs, motor mounts, exhaust, tires, radiator, etc. etc.

The price of the car reflected the need for at least one torque box and the "fixits", and I took the chance that I wouldn't run into any other major item needing repair or replacement on the car.


I had a few projects in front of the Avanti and didn't get to the torque boxes for several weeks. In the meantime, I did manage to tune it up and work on some of the "fixits". I also enjoyed driving it on sunny days. It drove and handled like a modern car. Tight, rattle free, responsive, comfortable and relatively quick (for an R1/Powershift).

Finally, I found the time to get it up on my friend Paul's lift.

The left (driver's side) torque box was definitely shot. It actually looked pretty good until we started poking at it with an ice pick and screwdrivers and beating on it with a body hammer. It looked like this when we were done stress testing it...

The right side, however, was in good shape. After the same stress testing, we couldn't find any weak areas. It looked like this...

A close inspection of the rest of the undercarriage confirmed that the springs, shocks, king pins, tie rod ends, and other suspension components were recently replaced as was the complete brake system, the exhaust system, the motor and transmission mounts, and the power steering control valve and hoses. The frame was "fish plated" in several areas, however, but the repairs looked fine and no other frame problems were noted.

New Parts

I ordered a new left side torque box from Classic Enterprises .

Classic makes high quality replacement sheet metal parts for Studebakers and other classic cars. Their Avanti torque boxes are their own 5 piece design which makes it possible to replace them without damaging the fiberglass rocker panels or having to lift the body all the way off the frame. Other manufacturers make a one piece box which most likely will work fine if you are doing a body off restoration.

With the Classic torque boxes you end up with replacements that are stronger than the originals due to the heavier gauge metal used and the engineering of the kit.

The parts arrived soon after I placed the phone order...

The main section is actually two pieces. The short front part is attached to the longer rear part with four screws. These will be disassembled for installation, then reassembled and welded together once in position on the car.

The other long piece is the part that will attach to the portion of the old rocker rail which remains in the car.

The other two fabricated sheet metal parts are the roll bar bulkhead and the end cap.

Also included are all the fasteners that are needed, adhesive/sealant, and instructions.

My friend Paul had graciously agreed to help with the installation. Neither Paul nor I had replaced the torque boxes in an Avanti, but Paul has had years of experience doing restorations and building hot rods and I was good at passing tools! We both reviewed the instructions that came with the torque box and an informative article from the Avanti Magazine by Chris Altenburg and Marty Burns.

Removing the old box/repairing seat hold down

We started by removing the driver's seat and the carpeting on that side of the car. Also the 4 nuts that hold the steering column to the dash, the body bolt in the well under the small cover below the steering column...

...the two body bolts in the trunk corners and the seat belt anchors. At this point, we should have also removed the bumper brackets, but we left that until the car was up in the air.

Next, we drilled out all the rivets that attached the old torque box to the floor...

...the rivets that hold the outer rocker rail were left in place since that piece of the old torque box remains.

As can be seen in the above picture (detail below), the nut plate to hold the left front corner of the seat was broken out and long gone.

This is also a common occurrence with many Avantis. Classic fabricates a piece that enables you to fix this from the cockpit, but with the torque box removed, it is possible to make the repair from underneath the car as original. We chose to do this once we had the old torque box removed.

We removed (or rather broke off) the old body bolts holding the torque box to the outriggers. Paul made quick work of removing the old torque box with the combination of an air powered die grinder and an air chisel.

The old torque box (or what was left of it) came out in pieces...but it came out.

No need to raise the body off the frame at this point.

Save all the shims and mark where they came from.

Leave as much of the old rocker rail in place as you can. Ours was pretty rusty, but solid enough near the top to give us good metal to rivet the new rocker rail to...

Next we drilled out the bolt stubs at the roll bar bulkhead and retapped the holes.

Now is the time to repair that torn seat attachment point at the left front corner

Paul fabricated a new nut plate using heavy gauge sheet metal...

We coated it with two part panel adhesive, then installed it from below, located it properly in the torn opening, cinched it into position using a bolt, then riveted it to the fiberglass seat riser just to make sure...

We left the bolt in place until the panel adhesive was dry.

Installing the new outer rocker rail

This step is pretty straight forward and simple. All the Classic pieces fit nicely and the rocker rail was no exception.

The rail was test fit into position against the remainder of the old one...

Once satisfied with the fit, we attached the new rail to the old one with three sheet metal screws to position it, then removed the screws and the rail and slathered the adhesive/sealant that came with the torque box from Classic to the old outer rail and the floor of the car where the "C" channel from the new rail would go.

The new rail was reinstalled and positioned with the same three screws in the same holes, then the rail was riveted in several places to the old rail and to the floor of the car from the inside and to the front of the rail through the wheel well.

One additional advantage of the Classic multi piece torque box is that this outer rail is now attached to the body with essentially a "T" section (original rail "C" section and new rail "C" section are opposite of each other) which makes the whole assembly much stronger.

At this point, we made the observation that torque boxes on these cars might be over kill. The doors of this car fit exceptionally well and open and close like new. This was/is the case with the rusted out box, with NO box, and with the new box. The car was originally ordered by and built for Mr. Morrison who was the founder and President of the Moulded Fiberglass Body Company...the company who built the Avanti bodies for Studebaker and the Corvette bodies for Chevrolet...so it may have received special attention. But, if you are counting on new torque boxes to fix sagging or ill fitting doors you might want to look for other causes of these problems.

Main box installation

In order to install the front and main boxes, the body must be lifted from the frame. We underestimated the amount that it needed to be lifted which dragged out the process some as we tried to fit these pieces several times...each time lifting the body just a little more. In the end, we determined it needs to lift about 2", primarily from the rear.

In addition to the body hold down bolts already removed (the two trunk bolts, the bolt under the steering column, the steering column from the dash, the seat belt anchors, and the bolts on the outriggers), you must also remove the bumper support brackets...all three supports per side.

With all body attachments removed, we used a porta-power at the roll bar bulkhead...

...then shimmed between the body and the frame so that we could remove the porta-power to install the main box.

The front portion of the main box goes in first. We had to lift slightly with a pry bar at the front body mount to slide it into place.

The main box goes in from the side, is slid forward to clear the rear spring mount, then back far enough to line up the front of the main box with the rear of the front box, then forward to fit the main box into the front box like it was before the temporary screws were removed holding these two pieces together, and into final position as a unit.

We found that we needed to trim about an inch off the vertical stiffener in the main box in order to be able to slide it forward enough to clear the rear spring mount.

We determined that the main box could either fit above the rocker rail bottom "C" section, or below it. We decided to fit it above it. This way, we could stitch weld the main box to the rocker rail. If it fit below the rocker rail bottom "C" section, we could only rivet or plug weld the main box to the rocker rail. Also, this way we didn't have to notch or cut holes in the bottom of the fiberglass rocker panel to accept the rivets.

With everything in position, Paul stitch welded the main box to the rocker rail and to the front box.

Roll bar bulkhead and end cap

The roll bar bulkhead was fit into the rear of the main box and positioned so the bolt holes lined up with those on the body/roll bar. The bolts were installed into the predrilled and tapped holes, and the bulkhead was welded to the main box.

The end cap was fitted to the end of the main box, then riveted to the body and welded to the main box. The small triangle filler piece that was removed with the old box was cleaned up and reinstalled to the new box with epoxy and rivets.

Finishing up

At this point, the shims were replaced in their original positions and the body was lowered back on to the frame.

To lower the body, we place the porta-power between the body and the frame rail near the traction bar attachment point with a steel plate between the body and the porta-power, raised the body slightly to remove the temporary shims and lowered it.

All body attachment bolts were replaced and torqued.

The main box was riveted to the floor by drilling through the original rivet holes.

Body seam sealer was used to fill the void between the fiberglass rocker and the bottom of the rocker rail. Also to cover the welded seams and to fill any other voids.

Paul coated the torque box with epoxy primer...

...and we covered that with rubberized undercoating.

There are several options to rustproof the interior of the torque box. One option is to do nothing. The original boxes lasted almost 50 years (maybe more) with no treatment. These cars will most likely not get the same exposure to the elements that they originally received.

Another option is to purchase stainless steel torque boxes from Classic for a slight premium.

You could also use a weld through primer on the inside of the box before you installed it.

What I chose to do is to drill several holes in the floor of the car with a hole saw in the center of the torque box and spray a wax based rust preventative compound (available at professional body shop supply stores) into the holes, then to plug the holes with plastic plugs. You could also add some closed cell foam if you wish.


It took Paul and I two days to replace the left side torque box. About a half a day was spent digging out three broken bolts...the two body bolts in the trunk and one that holds the bumper bracket to the frame rail. We also lost some time experimenting with how high to raise the body to insert the main box. Without broken bolts and with the knowledge we gained from the left side, the right side (if it was needed) would have probably taken 8-10 hours.

Paul's mechanical experience...even though he had never done a torque box...was invaluable.

In addition, it would be very difficult to perform this operation without a lift, air tools, the porta-power, a collection of spare nuts and bolts, and two full tool chests.

Bottom line is...I'm glad I purchased this Avanti and happy with the outcome of the torque box job. I gained some experience, had a great time working with Paul, and now have an Avanti that I am confident is solid all the way around.