How -to Replacing Unibody Panels

February 1, 2010 | By Larry Lyles

Certain Unibody Components Need Special Attention. Here’s How To Cut and Replace Unibody Metal.

IF YOUR CLASSIC ride happens to be a Charger, Mustang or Honda Civic, chances are you’re very familiar with the automotive term“unibody.”But if you’ve never had occasion to work on a unibody vehicle,let me explain a bit about the concept

The term“unibody”means the car doesn’t have a bolton frame underneath it. The body and frame have been assembled from the factory as a single unit.

In other words, the vehicle has been unitized.

If you will think back to when I did Project Charger and Project Mustang, one thing you didn’t see me do was remove the frame from underneath either of these vehicles. That’s because neither car had a frame…all of the suspension components were bolted to structural members of the body.

Think about how a soda can is constructed. It’s made of a thin tube of metal with reinforcing at the top. It doesn’t take much to crush one of these cans. However, if you open the can and add structural rings and bars to the inside, all of a sudden you have a very strong metal tube. No way do you want to try and crush this can against your forehead. Your head will break long before the can mashes. That’s how it is with a unibody vehicle. You have thin metal skins wrapped around extremely strong internal structural members.

That’s the basic definition of a unibody, but for our purposes what I want to address is where and how you can hack and cut on a unibody without the concern of destroying its integrity.

In the Days of Mild Steel…

I’m going to restrict this information to vehicles produced before 1980. What’s the deal with that? If you check your history books you will find that in the mid- ’70s the price of gasoline went through the roof and the domestic car manufacturers had to make some drastic changes to the way they constructed automobiles. On the way out was the car with a huge, heavy steel frame under it and on the way in was the lighter and even sturdier unibody vehicle that didn’t have a frame.

Did you know that neither Mercedes nor Toyota ever made a framed automobile?Truck, maybe; car, no. Thought you should know.

Getting back to domestic unibody vehicles, the boys in Detroit worked hard to catch up to the rest of the world as they lightened and strengthened their unibody vehicles. By the early ’80s Detroit had gone from using mild steel to make up many of the structural components of their unibody vehicles to using HSLA (high strength low alloy)steel.

HSLA steel is great in that it weighs less and is far stronger than mild steel. This meant less need for reinforcing and therefore less steel used overall, which equaled less vehicle weight and better gas mileage.

The catch to using HSLA steel is that if you do not have a piece of paper hanging on the wall that states you are certified and therefore qualified to make structural repairs to a vehicle constructed of HSLA steel you had better heed this warning and leave those repairs to the professionals. Why’s that? HSLA steel has very specific welding requirements in order for it to retain its strength and integrity.

With that out of the way let’s get back to early unitized vehicles, ones built before 1980 and made of mild steel, and talk about how you can safely make repairs to these bodies.

Compensating for the Overhang

I talked about this phenomenon back when I did Project Charger. Basically speaking, overhang is the weight of the engine putting stress on the unibody causing it to flex any time this type of vehicle is supported by jack stands placed under the torque boxes.

Where do you find the torque boxes and what do they look like? Illustration 1 gives you a good idea. The front two are found just inboard of the forward most tips of the right and left rocker panels and the rear two are found just inboard of the rearward most ends of the right and left rocker panels.

Appearance will differ from carto car, but for the most part you are looking at heavy metal boxes underneath the car that span the opening between the rocker panels and the frame rails. This is where unibody construction begins, by placing the torque boxes on measuring jigs to precisely locate the corners of the body. The rest of the unitized body is then constructed around these boxes.

Getting back to compensating for overhang, the engine must be supported in order to relieve any stress placed on the unitized body to prevent it from flexing. What if you don’t support the engine? The door openings are the primary areas to suffer. What will happen is that the openings will actually grow longer when measured from the upper hinge to a point at the top of the quarter panel and will narrow when measured from the roof straight down to the rocker panel. How much will the opening move? I’ve seen door openings stretch by as much as a half-inch. Try to put a door into that opening. It isn’t going to fit.

How do you compensate for overhang? My preference is to remove the engine and transmission as a unit. That ensures I’ll never have the overhang issue hanging over my head. If removal isn’t an option, you can support the engine with a hydraulic jack or suspend it using an engine hoist. How will you know when the weight and, therefore, stress is off of the body? The doors will open and close normally.

The Importance of Symmetry

Most unibody vehicles are built symmetrically. What that means is that if you draw a line perfectly centered down the length of the car any measurement taken from that center line to any point on the perimeter of the car will measure the same on both sides. That can be pretty handy when you are taking cross measurements in the engine compartment or underneath the car when swapping out a frame rail.

Here is how this works: If you take a measurement from the left corner of the core support back to the right rearmost upper fender mounting hole that measurement will be exactly the same when taken from the right core support back to the left fender mounting hole (Illustration 2).

Let’s say you cut out the rusty left fender apron on your Mustang and have the new apron clamped into place ready to weld. How will you know if the new apron is placed exactly where it needs to be? By taking the two cross measurements I just talked about.

When both of those measurements are equal, the new apron is in place and ready to be welded.

Will this trick work when replacing a frame rail? Yes, but you must take the cross measurements from underneath the car and you must get the frame rail height right. The height?Yes, let’s talk about that.

If you dissect a unibody vehicle and expose the frame rails, you can take what is known in the body repair business as datum line measurements. These are measurements taken from various points along the length of the frame rails down to an established point that in body shops is known as the datum line and in two-car garage restoration shops is known as “the floor” (Illustration 3).

Here’s a shade tree overview of how this can work for you.

With your project ride secured on jack stands, you can measure different points along the length of the left front frame rail,for example, down to the floor. Let’s say that measurement is 18 inches at the forward tip of the rail and 12 inches where the rail is welded to the left front torque box. That establishes a recordable and measurable height for that rail.

Next you take cross measurements going from the left forward tip of the rail back tothe right rail where it welds to the right torque box Let’s say that measurement is 42 inches. You confirm this measurement by switching sides and taking that cross measurement.

Then you remove the left frame rail and replace it with a new rail. To be sure the rail is placed correctly, you start by confirming that the new rail is precisely at the two height measurements taken from the old rail.

After that, you can take cross measurements and when those measurements equal the measurements taken before the old rail was removed, the new rail is precisely placed and can be welded.

Don’t Section Your Rails

What I’ve given you here is information body shops use to make accurate repairs on crashed vehicles. You’re lucky in that the project ride out in the garage most likely hasn’t been crashed; it’s just rusted. What that means is that you can take this information, apply it to your ride by compensating for overhang, taking accurate measurements before you remove any of the rusted metal, then use those measurements to precisely place new metal back where the old rusted metal was mounted.

So, now that you are ready to install some of that new metal, let’s talk about some other things you should know.

Let’s start with the front frame rails. Never, ever attempt to section the front frame rails. Section?Yes, that’s when you decide to take the easy way out, cut the rail just ahead of the strut tower and splice in a new piece.

Why the “no splice” rule for front frame rails? I’ll not go into how sectioning these rails alters the crash characteristics of the vehicle because that subject is too deep to discuss here. I’ll just say that the integrity of the front suspension depends upon the integrity of the frame rails and hacking and cutting on the rails will affect the integrity of the entire structure.

On a Mustang, for example, you can replace the outer rail, the inner rail, or both halves. Relatively speaking, the parts are cheap. My recommendation is to replace both halves all the way back to the firewall.

Some front frame rails are available only as complete units. That’s always a box structure and quite often these rails extend some distance under the front floor pan. That means more spot weld drilling and more welding, but the result will be a first-rate repair.

So, this procedure involves taking precise measurements before cutting, drilling out the spot welds for removal, cleaning and rust treating before assembly, then making the new spot welds where the factory originally placed them. There is no need for additional bracing or supporting beyond what we previously outlined to prevent the body from flexing as you cut out and replace front frame rails.

Interior Floor Pans

If you are replacing interior floor pans, you don’t need any bracing inside the car. Floor pans are no different from any other non-structural panels on your ride. I like to remove the doors if for no other reason than to provide better access to the interior.

Take the time to measure, mark and pre-drill new floor pans in preparation for swapping out seat belt fasteners and seating mounts.

Of course, it goes without saying that the interior should be gutted before you start work. If you have been reading my stuff for any length of time, you may recall my mentioning that interiors don’t do a slow burn when hit with a welding spark. They go “poof!”

Trunk Floor Pans

These pans are no different from interior floor pans. They can be replaced without additional bracing to the body. The same goes for the rear frame rails. It is rare that the rear frame rails will need replacing without the trunk floor pan needing replacing. Replacing both at once is the best route to take.

To qualify the last paragraph, rear frame rails almost never need to be replaced in their entirety. In fact, most rear frame rails for unibody vehicles come in either full-length rails or in rear sections. In most cases you can opt for the rear sections and not have to install the entire rail.

To qualify these statements even further, trunk floor pans and rear frame rails should always be replaced with the quarter panels and rear body panel in place on the car. The quarters and rear body panels add to the rigidity of the unibody structure and will help ensure that the body doesn’t flex any time the rear frame rails are removed.

Need to replace the rear frame rails with the quarter panels off of the car? In that case refer to the information I gave you on replacing the front frame rails. The only difference is that you will be working at the rear of the car and not at the front.

So it is OK to section a rear frame rail? Yes, the rear rails are not subjected to the same stresses and forces that are brought to bear on the front frame rails.

Replacing Rocker Panels

Rocker panels present a whole new set of problems. Recall that I said the torque boxes are connected to the rocker panels and that helps establish the basic structure of the body? What that means is that if you attempt to remove either of the rocker panels without first bracing the body, it could flex out of shape. If that happens, you’ll need some really deep pockets and a highly skilled body technician to bring the body back into shape.

Let’s look at how to brace a body to replace a left rocker panel. Bracing starts by removing the door. You cannot efficiently remove and replace a rocker panel with the door in the way. That said, you have to be sure the door is going to fit back in the opening after the rocker panel has been replaced and before any serious welding has taken place.

How do you brace the body? By building a ladder-style brace and welding it inside the car just inboard of the door opening (Illustration 4). Why a ladder-style brace? Nothing you do to the car is going to rack or otherwise alter the positioning of a ladder-style brace. Build this brace out of one inch-square steel tubing and weld it solidly to the inner door post and to the inner quarter structure.

What size brace is best? That really depends upon the vehicle. The ladder can be as narrow as six inches or as wide as two feet. It can be as long or short as necessary. It just depends upon where you think the best places to weld are located on the door post and quarter panel inner structure. The goal here is to span the door opening to ensure that the cowl and rear body structure remain rigid while you replace the rocker panel.

Now, let’s assume the body has been properly braced and you are ready to remove the rocker panel.

Let’s also assume that you have a new rocker panel in hand. No? Never hack and cut on any part of your ride without the replacement panel in hand. That’s the rule.

Now, let’s consider how much of the rocker you are going to replace.

Rocker panels come as “outer” rockers and “complete” rockers. The difference is that the “outer” rocker is just that, the outer skin. It does not come with the “inner” rocker

Generally the “inner” rocker is a flat strip of metal that welds to the back of the “outer” rocker, to give it that boxed shape, and to the floor pan. This box shape is what gives the rocker its strength.

Note: Some “inner” rocker panels may actually be part of the floor pan.

The first order of business is to take some measurements (Illustration 5). Even though the body has been thoroughly braced and there is zero chance it will flex or move out of shape, you still need to take some measurements to be sure.

Notice in the illustration that measurements are taken horizontally, vertically and diagonally across the door opening.

All of these measurements are strictly for insurance purposes and they should be taken before removing the rocker and again after the new rocker panel has been screwed into place and is ready for welding.

What about replacing only a section of the rocker panel? This is a perfectly safe procedure, and quite often it’s the more preferable choice, but only if it’s done correctly.

To section an outer rocker panel you must add an overlapping backing panel to the inside of the original rocker in the area to be welded. Notice in Illustration 6 that a section has been cut out of the original rocker panel. The new replacement piece will be laid in place and butt welded to the old panel after the insert tabs have been slid inside the original rocker and the replacement piece and spot welded in place. The tabs are made of three-inch-long sections removed from the ends of the replacement panel.

One of the secrets to making this type of repair is to cut the replacement section piece 3/8-inch shorter than the length of the piece being cut out. This allows a 3/16-inch gap at each end of the new section where the piece is butt welded to the original rocker panel.

More Unibody Tips

Here are some other things you should know before replacing metal on your unibody vehicle:

Floor pans, whether interior or trunk, can be changed while the body is on a rotisserie without having to add bracing to the body.

Frame rails, both front and rear, must never be removed from a unibody vehicle while that body is mounted on a rotisserie.

Rocker panels must never be replaced while the body is on a rotisserie even if the body has been properly braced using a ladder type brace across the door openings. Why not? Despite having braced the body, the rocker panels can still bow due to the stress placed on them.

Got a question? Send it along.


LPL Body Works, LLC

5815 Contented Lane

Amarillo, TX 79109

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