Thursday, June 5, 2014

Rear Valence & Trunk Lid Fitting

First, I offer my apologies to the followers of this blog for yet another long period of silence between updates.  Sparing the details, it’s been a rough past six weeks for our family and it was time to take care of our own for a while and, as such, the Boss project rested patiently until things calmed down enough to resume.  However, I am VERY happy to say that there has been some fantastic progress in recent days and a good bit more to come in the next few weeks.

As I have mentioned in past posts, I like to begin the body fitting process from the rear of the car.  The main reason for this is the rear quarters are the “anchor” panels that every other panel eventually keys off of for final position.  Therefore, it is critical the back end of the car is tight and right before moving forward with other panels.

Having finished the “basic” bodywork on both rear quarters, I was able to concentrate on fitting the rear valence panel and getting the end gaps just right, followed directly by fitting the rear deck lid.  But, first things first…..
About 25 years ago, I managed to get an NOS rear valence for the Boss and squirrel it away for the time when I would fit it to the car during restoration.  So naturally, I figured this would be the perfect place to start actually fitting a body panel to the car as it would probably be a slam dunk for fit.  WRONG!

The more I tried fitting the stock NOS rear valence, the more I didn’t like it.  It was rather poorly made overall, the edge finishes were crap and the edges themselves look like they were formed with a baseball bat.  Complicating things further, the panel width was roughly ¼” too wide for the car!  So, after a short debate with myself on what to do, I added a reproduction valence to my NPD shopping list and moved on to evaluate my trunk lid options.
I have to admit, I was only marginally confident that my production trunk lid was going to be usable after all of the abuse it had endured and the rust it had accumulated over the years in the corrosive South Texas Gulf Coast air.  The first thing on the evaluation agenda was to bead blast around all of the perimeter flanges to see how solid they were.  Much to my surprise, they were all in excellent shape after a moderate round of blasting.

However, when I switched my focus to the “beauty” surface of the deck lid, the wheel rather quickly fell off the wagon.  In only a few minutes of light blasting it became painfully evident I was not going to be able to use this lid as there were about 6 significant pinholes that were revealed when each of the heavier rust freckles was removed by blasting.  Instant game-over as far as I was concerned.  Another significant addition to the NPD shopping list and plenty of justification to make a trek to their Canton, Michigan warehouse for a parts run!
After carting my new, virgin sheet metal home, I set about evaluating the fit of the new valence with excellent results.  Although rare in my experience, I am happy to report the reproduction valence fit miles better than the NOS piece ever did and made for much less work in order to get the corner gaps fixed to my liking.  Not a bolt-on by any means, but pretty darn close in comparison to the stock panel for sure!

After a solid day of fitting, grinding, fitting and grinding, I was able to get the end gaps looking very sporty and infinitely better than 90% of the restored examples you regularly find.  Once I got the gaps very close to perfect, I sealed the edges with a careful application of silicon bronze along the entire “worked” edge to ensure there would be no compromised edges on either end of the panel.  Then I cleaned up the edges with a surface disk and indexed the panel to the body to ensure I could always repeat the fit each time I needed to remove and reinstall the panel.  In the end, a thin application of filler to the joint area and the valence ends will be able to be perfectly mated to the body contours.
Next up, it was time to test fit the new reproduction deck lid I picked up.  Om first blush, the panel is a very nice quality stamping with features almost perfectly mirroring the stock lid.  In fact, for 90% of the restorations out there, it will likely be as close to a bolt-on affair as can be expected.  However, for the demands of a project like this, I knew there would be many hours of work required to get it to fit at a much higher level of precision.

After quite a bit of manipulation and adjustment, I actually managed to get the side gaps almost perfectly where I wanted them and the lid fitting surprisingly flush around almost all edges.  However, the leading (front) edge of the lid had a rather massive gap that simply could not be remedied with any amount of adjustment.  The bottom line was that there simply wasn’t enough material along this edge to achieve a proper gap and the remedy was to employ the rather standard custom car practice of adding material to the leading edge of the lid to close up this gap enough to allow careful hand-fitting of this edge to the trunk opening so that the cap could be much tighter and precise.  But, before diving into that work altogether, I just had to mock up the tail of the car just to get a look at what it will look like all assembled.  And what do you know?!  It actually looks like I might just have a bloody CAR after all and most indications suggest it might just be a 70 Boss to boot!  We are back on the move!

This 25+ year old NOS rear valence was supposed to be the hot ticket to start the rear panel fitting.  Unfortunately, I didn't like how rough the finish was one each end and how poorly the fit was overall.  Didn't take long to decide to pick up a new reproduction valence to see how the two would compare.

Straight out of the box, the new NPD-supplied repreoduction rear valence was miles ahead on fit over my old NOS piece.

Still a bit too wide overall, but the new reproduction valence will work quite nicely.

Left side was the better fit, but both sides will be getting the custom gapping techniques applied.

Like almost every Mustang rear valence, these punting bracket tabs will need to be cut back beyond the edge to allow proper gap work to continue.  The cut lines are marked in yellow paint pen in this shot.

First steps in establishing proper gap create a slight separation between the rear quarter corner and the valence.  Already, the fit of the panel looks far better than the bolt-on fit.

A little more work on the right corner and things are starting to shape up nicely.  Still a bit tight, but definitely better.

Left side corner is also tight to start but quite a bit less work will be required on this side to get the gap correct.
After sealing up the edge of the panel with a bead of silicon bronze weld, the gap is hand dressed to the perfect width, creating a subtle, yet unmistakable fit quality that no factory Mustang enjoyed.

My old factory trunk lid conspired to disappoint when I discovered about 6 small pinholes in the outer skin as a result of the murderously caustic South Texas salt air environment.  Dammit!  Luckily, the much-improved replacement trunk lid from NPD proved to be a worthy replacement and will involve far less work to fit to the body than the repaired original would have allowed.

In the mock-up phase, I add a strip of masking tape to help visualize the overall end cap and trunk lid alignment.

I was genuinely surprised at how well the new trunk lid fit right out of the box.  Deck lids on 69-70 fastbacks are quite finicky to get the gaps right as it is, but the NPD lid, while not perfect, is a very nice piece of business.

Factory hinges and adjustment range allowed the right side edge gap to be almost spot-on with no modification.

A little extra time in adjustment had the left side gap quite acceptable as well.

The gap at the leading edge was another story.  While it rather accurately reflects the wider gaps in this area of a factory panel, the look is just not something I can live with.  We will need to close up this gap to the standard 3/16" spec with quite a bit of work that will be covered in the next installment of this blog.

I just couldn't resist the temptation to see what the rear of the car might look like with all of the panels in place.  For the first time in YEARS I can actually see that this is a 70 Boss Mustang!


  1. Hey Sven, no apologies necessary, especially when it comes to family. I know where you're coming from. For me it's God, family, and then whatever else comes like the Mustang, flying, and the Tigers.

    I feel your deck lid pain. What is it about Texas cars and rust in deck lids? I will say for aftermarket parts they certainly fit well. I'm surprised about the NOS valance fit. I guess I'm fortunate I have the original one that came with the car. So hopefully it will go back together OK.

    The Boss is looking good...real good! I'm waiting for the deck lid mod post to see what you do with it.

    1. Thanks for the comments Dennis! It really broke my heart when the factory lid ended up a non-starter. In the end it will all be fine, but I really had my fingers crossed. You are correct that the aftermarket lids are pretty impressive. They certainly didn't used to be that way! I credit NPD for really driving the improvements in these particular pieces.

      You're gunna like how we go about cleaning up the fit of the lid in the next entry!

  2. I was of course wondering what happened to you, good to know that everything is ok in the end. Some things really do take priority.

    The work is looking really good, and things are definitely coming together. This a really good example of how sometimes the original pieces aren't as good as you might believe. In this case NOS didn't mean better. It is really good to know that the aftermarket has really stepped it up recently to give the better parts that we as restorers demand. Can't wait to see more!

    1. Howdy Grant! You are most certainly correct about priorities. Every once in a while I get a bell-ringing reminder from on high to keep that in perspective.

      While I still approach every aftermarket part with considerable caution, this was one definite exception and I am very glad for it!

      Stay tuned!

  3. Nice work Sven! I'd love to see a "how to" for your silicon bronze technique for panel edges. I can see from your valance work that my own leaves a lot to be desired. I may need to rework it when I replace my deck with a new after-market part. You continue to motivate even while I'm supposed to be "done"! ;-)

    1. Hi Alex and thanks for the comment. I will try to better document the silicon bronze use as I go through the exercise of closing up the body gaps around the car. The technique I use requires a TIG machine with very good low-end arc stability as essentially, this is a TIG-brazing technique that requires no flux. Having said that, NEVER use a brazing flux or flux-coated rods for this type of work as you can never get it perfectly clean and rust will undoubtedly return to the party wherever the flux was deposited (and don't ever let anybody tell you different!).



    2. Thanks for the tips. I was (incorrectly) assuming you had used brazing equipment. Anxiously awaiting your how-to.