More Mustang unobtainium — Trunk corners

If you own a coupe or convertible First Gen Mustang you know that the trunk corners are not reproduced, so if yours are in bad shape you either repair what you have or you find others that are in better shape.

Left QuarterOn my car the corners had already been repaired once, and not very well. The lower half had rusted away at some point and someone along the way welded some new metal in. The problem is that it was not shaped right, nor really aligned with the rear quarter. So the old quarters with the patches were simply bashed to conform to the patched corner. It was a bit of a mess.

When I replaced the quarter panels, I left the tail panel on the car to help make sure the new quarters were square and lined up with the rest of the car. Since the new quarters are now on, It was time to remove the old tail panel and replace it with the new one. The tail panel is attached to the quarter panel and to the bottom of the trunk floor. In my case the tail panel had been replaced several years ago, but was installed without any paint, only the e-coat, so surface rust was getting in everywhere. Additionally it was not really welded in very well, so it was actually pretty easy to remove.

With the tail panel off the car, I started by removing the driver’s side corner. My first attempt at a fix was to recreate the missing section of the corner. You can see in the photos I got pretty close. So I cut off the really bad section and tried assembling it for a test fit. It fit Okay, but not great. As you can see the part is quite complex with lots of compound curves and creases. I thought about making a wooden buck to hammer the shape out. But this was proving very cumbersome.

Finally I decided to mock up the finished location for the taillight panel, and see what I would really need to have the corners repaired.

_MG_0425Using clecos I reinstalled the driver’s side corner section. Then I clamped the tail panel to the trunk floor. Next I reinstalled the trunk lid and aligned it with the quarter panels and the deck lid filler panel. This is where it got a little weird. I need to move the tail panel out almost three quarters of an Inch to line up the flat part with the area in the trunk where the gasket sits. I double checked the measurements and everything seemed to line up. So I’m not sure if it is some difference between a 69 and a 70 tail panel, or what. Because both the new Fiberglass trunk lid and the original trunk lid need the tail panel out that much. Also it fit within the quarter panel end caps correctly. This is strange, very strange indeed.

On to the fabrication.  At this point I decided it may be easier to just try to make my own trunk corners. I have not talked to anyone else mixing 69 and 70 parts; maybe this is one of those areas where they are slightly different.

Grabbing a sheet of my trusty card stock, I made a template of the outer edge of the corner from the remains of the original corner, and my hand made patch. After checking this for fit in the quarter panel, I again assembled the tail panel, and aligned it with the trunk. As you can see in the photos, I marked up the cardboard where the bends should start and where they meet with the tail panel. This gave me a rough template for making the parts.

From this I cut out a slightly less crude cardboard form, and again ran trial fits until it was a good shape. I used a lot of tape to hold the bits together at the needed locations. At this point I had the part that attached to the quarter panel ready, so I felt I could commit that to steel. I traced the flat portion out on some 20G sheet steel. Then cut out, and trimmed to fit correctly.

Now using those parts (steel to quarter panel, cardboard to the tail panel), I built what would be the final templates for the corners. This process is really three steps. 1) mockup, 2) measure to check fit and placement, 3) repeat. As you can see in the photos below this leads to a continual progression of refinement, up to the finished template. The final shape to give me the correct angle and radius on the top corner, turned out to look somewhat like a hockey stick. Sharp eyes will see a fastback corner there on the bench. I ordered one to see if I could use even part of it, but they are so different from the coupe that pretty much none of it is the same.

Since my hand made corner is nothing like the original I did not try to mount the tail panel in the same way. If you look at first pictures, the original corner has this complex double radius curve that matches the 69 concave section. I was not about to try to reproduce this. The 70 panel is flat, but still has the extra curves for the mounting section. I’m pretty sure this came about because they wanted to reuse as many components as they could. My guess is that they would have built a flat mating surface to match the 70 tail panel if they had decided to build a new corner.

For my corners I decided to have the metal mate with the flat portion of the 70 tail panel to make it easier to align, and much easier to fabricate. If you look at the original parts there is an amazing amount of adjustment in all three dimensions for locating the tail panel to the car. This wide range of alignment space really helps when you’re fabricating your own parts. I got my corner to the critical depth to allow the trunk and quarter panels to square up, then checked that the trunk gasket will mate with the tail panel.  After that the rest of the locations where pretty much mandated. Once again I transferred the cardboard templates to steel.

I cut out the hockey stick shaped part, and using several different diameter pipes, and dollies , I worked the steel into the correct shape. To ensure a good fit, I used the interior cutout on the flat plate that mates to the quarter, since it was trimmed to give the cardboard the right shape, and position. I did cut the hockey stick about 1/2” long to give myself some adjustment space left to right. Next I bent the lips that would attach the two parts together. You’ll note in the photos that I cut the corner sections, rather than use my stretcher, this is because the radius is rather tight, and the lip is short, so the amount of stretching would have caused thin areas, and likely tore the metal anyway. Since this will be on the back side and welded up, it will never be seen.

And the two become one. Now that I have two parts that look rather good, it’s time to mock up for a final time. This time I mocked up a lot more. Along with getting the tail panel centered, and aligned, I also installed the tail lights, and the quarter panel end caps. This was a good thing as where the corner sat on the flat of the tail panel caused an interference with the tail light bucket. Once this was discovered I move the tail panel from the car to my work bench to allow better access all around. After a lot of hammer and dolly work, I was able to have a good fit between the tail light bucket and the trunk corner.

I also wound up trimming the flat part an additional 3/16 of an inch on the inside to move the hockey stick part closer to the out side of the quarter panel. This provided the needed clearance for the tail light bucket. I was glad I had left about 1/2 inch of material as I need some of it.

With the left one fitting nicely, I used it as a guide to build the right side. As you can probably guess the left side took me about 20+ hours to build, the right side only took me about 4. now with four loose parts, I was ready to assemble them in to a left and right corner. The reason I did not start welding until now was that I wanted to have enough adjustment in both sides to have the tail panel fit. It was much easier to clamp and cleco everything together, and then tack weld it.

In the next set of photos I show the progression from tack welded to fully welded. It’s a pain to tack them together in the car, but It was the best way to ensure the all the parts were in the right position and did not move around on me. Once the parts were tack welded, I could remove them from the car and fully weld them up.

As you can see it really looks nothing like the original; however it does fit correctly, I’m off about one sixteenth of an inch on how far the tail panel is away from the quarter, and about three thirty-seconds of an inch side to side. So over all well within the 3 sixteenths Ford has specified.

Here are the finished corners, and tail panel.

Here are the finished corners, and tail panel.

Next time, I cover how I aligned the tail panel. Then after that we will make the top corners of the trunk. Yes you read that, those are not available either.. but I’ll show you a trick that works well.