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!