Body Repairs - New Chassis Installation

While i'm making sure that the rear of the car has been structurally secured, I can start to measure up and weld in place the box-tube that will form the "full chassis". On a standard Mustang, the area where the rear torque-boxes get welded to the inner sills and rear frame, forms a nice hollow void. This "hollow" is right in front of the forward mounting point for the rear leaf-spring. This provides the perfect spot to insert a short piece of box-tube that will connect the rear frame-rail to my new custom length of chassis tube I am running inside the sills. Here is a diagram of where I am adding chassis material to the standard Mustang body.

The areas highlighted in red are the chassis additions (box tube). Tube dimensions are as follows: sill area and sill connection to the factory standard frame rails is 2" X 4" box-tube, sub-frame connectors are 2" X 2" tube. All tube is 2mm wall thickness. The total length of tube added is 3.0 meters for 2" X 2" and 4.8 meters for 2" X 4" tube. This adds just 18kg to the whole body - but improves its torsional rigidity out of site!

Here are some shots of the "sill chassis" being fabricated. With the body shell flipped over you can see the new rear frame rail tack-welded in place and the old inner & outer sill have been completely removed (drilled out the spot welds). As well as removing the bottom-front edge of the old rusty wheel wells....

I fabricated the inner sill out of strips cut from a separate length of 4" X 2" box tube. The gap you can see in the strips is intentional - it gives me a neat gap in the middle of the inner sill (looking from iside the cabin) for welding to the box-tube. This means the box tube is secured to the inner sill not just along its top and bottom edges (when looking from outside the car) but along the middle as well. Very strong!

The new outer sills fit nicely over my new "inner-sill & chassis rail combo", except where the outer sill "necks down" at the front of the car - see below. So I cut the lower edge off the front lower part of the chassis rail, spun it inwards and then welded it back in place. The outer sill now hides my new chassis completely. For the record, the new inner sills are 155mm in height.

Here is a shot of the rear of the sill chassis showing the stitch welding securing it to the inner sills. This is the rear section of the new chassis rails that weld to the front sping mounts (front mounts of the rear leaf springs).

Now (below) you can see the new chassis rail sitting in place with the front edge "inverted" so the outer sill will sit snugly over it. This was just one of many trial fittings.

The shot below shows the new inner sill in place on one side of the car. It is a vast improvement over the old patched-up sills I cut out. In this shot you can see the old rusty sping support & lower torque box still in place on the near side of the car. The new sills will be a perfect foundation to weld the new floor pans to. Check out the holes in the old frame rail at the bottom of the picture!

Where there is a 90 degree bend/join anywhere in the new chassis, I was originally intending to include extra metal to support the join. But there is no need to add the extra metal - it would just be adding redundant weight. With the front of the rear-suspension mounting points welded directly to my new chassis rails, I will nicely distribute the forces being pushed thru the rear axle as evenly as possible throughout the entire car.

This is especially important considering that the big-block up front is heavy, despite being on a diet of various aluminium goodies. Its weight will resist movement when the car accelerates - simply because of its momentum. I have therefore paid special attention to how the new chassis elements mount to the front frame-rails. Any connections pass thru both walls of the existing Mustang front-rails. Welding the new chassis onto just one face of the rail will not distribute the loads evenly and fatigue cracks would eventually occur.

The next few shots shows how I connected the 2" X 4" "sill chassis" to the front frame-rails. The connecting piece runs at an angle to the frame rails to promote strength but still allows plenty of room for larger tyres to fit. This angled connection should not only transmit acceleration forces cleanly to the front of the car but help control twist in the body shell too.

Firstly - measuring about a hundres times before cutting the holes in the frame-rail. The holes in the inner & outer walls of the frame-rail need to be offset from each other as the connecting piece comes in at a slanted angle (not a right-angle) as mentioned. This was tricky to get right!

Trial-fitting the connecting piece.....

Welded in place.... and a perfect fit.

Other side of the car now done too.

Photo from inside the engine bay showing the welding done to complete join.

This shot gives an overall view of the new chassis in place while I "trial fit" it with the outer sills removed. I'll soon be able to remove some of my "space frame" (to install the new floors, etc) as the new chassis is tieing everthing together again.

Below you can see the chassis finally welded in place with sills included.

And below is a shot showing what actually resulted - based on the original drawing I made (at the top of this post). It hasn't varied too far from the drawing. You can clearly see the zinc-coated box tube as it protudes from the front of the sills and connects to the front frame-rail, as well as the rear "stub" that connects to the rear spring-mount.

Not much chassis flex now!

Over and above installation of the "sill chassis", I am also installing the more conventional "sub-frame connectors". As I am replacing the floors and the front frame extensions, it is relatively easy for me (by way of the rotisserie) to install fully welded connectors. These will really tie things together and give me two protected channels within which I can run fuel lines and battery leads from the trunk area up to the engine bay of the car. But rather than weld the connectors to the front frame extentions via pieces of 3mm plate, I am inserting the connectors inside the rail extentions and then welding them in place. Fabricating things this way gets me two nicely protected tubes running from the rear of the car all the way to the engine bay.

Once the chassis is complete i'll be able to install the new wheel-wells (you can see in the photo's that i've alreay started them). The rear panels & wheel wells will probably be the next chapter.

Body Repairs - Rear Frame.

Every person that restores an old car gets some sort of surprise at how much more work there is in a project than they initially think. Very occasionally this is a pleasant surprise (less work) - but mostly not. Once I spun the car over I could fully see the extent of rust damage to the frame - the frame I was hoping to use to connect my new chassis elements to. Checkout the old welding repairs on the rear frames in these shots........


And yes... I had to angle-grind through the old sagging leaf-springs to get them (with the diff) out. Apart from severely rusted threads, the front leaf-spring bolts could not be undone as the bolt/nut heads were rounded off.

You can see the nice big rust hole in the frame rail right where the leaf-spring bolts up.

It was soon apparent that i'd either need to graft in full-length reproduction rear frame-rails or make up new frame rails from sheet steel. The cost of each repro' frame-rail is about $300AUD - so $600 bucks in parts (before any labour) would be needed if taking this option.

This seemed expensive so I did a quick internet search of specifications for "automotive" structural steel sheet and found that CA260S-G or CM350-G specification sheet were the likely candidates for me to build the rails from. These steels have the tensile & yield strength I am after, are great to weld and Bluescope Steel make the CM350 in the 2mm (80thou") thickness that I needed. But finding a supplier that could get me just one sheet is almost impossible! I could easily order it in 1000Kg lots, but just a few pieces is a big problem.

Something else that needs to be considered is the safety requirement to have frame-rail "crush zones" so the Fastback will actually deform in case of a serious collision. Weighing all this up with the added (many) hours needed to fabricate rails from steel sheet - I have elected to go for the reproduction frame rails. Now I "only" have to remove the old rails and graft in the new ones - still a lot of work!

The rear quarters, sills and tail panel were the next items to look at. I've described in a previous post how I found that both rear quarters had been replaced and pop-riveted in place.... well here are some shots of uncovering the joins and removing both old quarters.....


Check out the pop-rivet holes (below) where the quarter meets the sill! I had to drill out about 50 rivets all up on each quarter. I simply used an industrial wire-brush in the 9" angle grinder to remove the body-filler covering the rivets...

I used the new (2 piece) rear quarters as a template to mark and then cut away the old quarter panels (see photo below). I'm hoping to only use the main piece of the replacement panel -thus not use the second piece that incorporates the rear vent and outer rim of the rear window frame. I marked and then cut in a way that matches the new replacement panel but initially gives me about an inch of extra panel on the car so I can trim things back later if needed.
In the shot below (with some of the outer sill cut away) you can see patches on the lower-forward part of the inner sill. Thankfully, new box-tube welded along the inner sill (to create a hidden "full chassis" once the outer sill is put back) will strengthen this whole area up considerably. But the new chassis work will be in another post...


With the outer sills and rear quarter skins removed I can start to see what it will take to fabricate and add the new outer chassis rails. But I first need to install the rear full frame rails so I have a good foundation for the new chassis rails to connect to. My box-tube "space-frame" that is supporting the bare body needs to be suplimented at the rear to ensure there is no sag when the rusted rear-frame rails come out. So I added some support brackets and welded them from the space-frame to the body at various points (up under the back of the rear window, to the rear tail panel support, wheel-arch supports, etc). You can see where i've removed the old tail panel below. It give you great access to work on the rear-frames and wheel arches. But you need to be careful not to cut away the alignment holes in the tail panel support. These will be used to correctly centre & re-attach the tail panel.

I then drilled out the spot-welds in the floor & boot panels where they attach to the rear frame-rails, or, I simply cut out floor & boot panel with the frame-rail still attached - if it was rusted thru and needs replacing anyway. Keeping the old rear quarter panels, tail panel, front-seat platforms, etc gives me some original metal to use when putting in any required patches. The previous owner had at some stage replaced the inner rear guard panels - the ones that wrap up and over the rear frame-rails and form the outer floor of each side of the boot. So while these are salvagable, just about everything else that attaches directly to the rear-frames has to go. The rear-frames I purchased are from Dynacorn and each one includes the frame-rail itself, front & rear leaf-spring mounts, rear exhaust mounting nuts, rubber stop for the diff and even the outer seat-belt threaded mounts. While they are quality units and will provide a great foundation for the new chassis to weld to, I found one of them was mis-aligned when it was welded on their jig. I had to cut thru a few spot-welds, square the front spring mount up and re-weld it in place.

The rusted wheel tubs are being replaced by cutting out what's left of the inner & outer wheel arches but leaving the central "spine" of the original wheel arch in place. The "spine" (just my word for it) is simply where the inner & outer wheel arches are welder together to form a spine - which is then welded to the body shell up under the Fastback rear vents. You can see one new frame-rail in place below. Note the gap between the new rail and the inner wheel arch - this is where new sheetmetal will be grafted in.

Leaving the spine in place gives me a good template to follow with the new sheetmetal. The inner profile of the finished wheel arches I am making will be much more square in cross section when compared to the original wheel-well. The original wheel-well is formed by using pressed wheel arch halves - great for skinny tyres shaped like doughnuts but no good for whide rims with low profile tyres! The new wheel tubs will give much more room for the modern (square profile) wheel/tyre combo I intend to use.

Here is a shot of how bare the shell looked as I got close the end of the removal of all the "rusty bits". I am thankfull I built such a robust space-frame, otherwise the shell would have collapsed on the floor!

And finally, here are some shots of the new rear rails tack-welded or sitting in place. You may notice the following in these shots........

• Modified frame rail (on the right of picture) to fix dodgy welding & bent spring support.
• Use of 1/2 fibreglass rod to make sure both rear frames line up with the Mustang body.
• New sill chassis components sitting in place within the new outer sills (not welded yet).
• One outer wheel arch removed (just begining preparation for new wheel tubs).
• New rear quarters in place as an initial trial fit (they need to be lifted at the rear).

Just in case anyone is actually following this saga, I am well into the creation of the new chassis. But I need to actually finish it before I put up the next post. Hopefully in a week or two i'll be done as I have a long weekend coming up that should see me spend some time on the car. I would value any comments anyone would like to submit (they don't get displayed - unless you request it), I need all the encouragement I can get at this point!