Well, I didn't feel like I got enough done last weekend, so I went out to the garage and did a little work each night this week.
So, the basic idea for the blowout straps is, I want to run two straps down the length of the back window.
Like I said in an earlier post, blowout straps are a common feature of race cars that are based on production cars. When the car is at high speed with the side windows down, it is possible to generate a pressure difference across the back window that can be sufficient to suck the window out. My car won't really need them (I hope?), but I thought it would be a cool design cue to play up the race car angle.
But, how to mount them? From what I've seen/heard, on the old race cars they'd typically be mounted with rivets or sheet metal screws. That's fine for a race car, because they don't have to worry about aesthetics or rain water leaking into the car or anything like that. I wanted to work out a better way to do it, though. Something that would look nicer, and most importantly not leak.
The idea I came up with was to use a threaded standoff. I ordered four zinc-plated standoffs from McMaster-Carr.
The standoff is threaded all the way through, but my plan was to weld one end shut, so that it would be blind. I figure that with the end welded shut, and with a thread locker and/or thread sealer on the fastener, there should be no way for water to leak through.
McMaster-Carr has a very searchable/sortable website, which made it easy to find what I was looking for. In picking the standoff, the main thing I wanted was a 3/8-16 thread, and it turned out that the hex-shaped, zinc-plated standoffs were the best option available for that. They also had stainless steel, but I didn't want to be trying to weld stainless to regular steel.
So, for starters, my buddy Allen helped me mark Matilda's centerline last weekend. You can see a couple of the marks in this picture (a long one on the trunk lid, and a very small one at the rear window molding).
We made one mark on the trunk lid, one mark at the base of the rear window, one mark at the top of the rear window, and then checked with a straight edge to verify that all three marks lined up with each other, and with the hole for the lock in the trunk lid.
Next I picked a spot that looked like a good place to put the blowout strap. I just did this by eye. After years and years of looking at photos of old race cars, I felt like I had a pretty good feel for where it should go.
So, I measured the distance from the centerline to the spot I'd picked, rounded it off to the nearest inch to make it easy, and then marked it, and marked a corresponding point on the other side of the car. I spaced the marks an inch and a half off of the inner edge of the rear window molding, and fourteen inches from the centerline of the car. Then I centered a standoff on each mark and traced around it, to show the hex shape that I would need to create.
To start making the holes, I took a Phillips head screwdriver to use as a punch, and a hammer, and made a small punch mark at the center of each mounting point. Then I drilled it out, stepping up the drill bit sizes until I didn't have any larger drill bit.
From there, I took my Dremel and started milling the hole out to match the hex shape that I'd traced on to the car. I went slowly and kept checking size of the hex hole compared to the size of the standoff.
When it felt like the standoff was almost ready to fit into the hole, with just the slightest interference, I took a hammer and tapped it in. My idea was that I wanted it to fit tightly enough that it would stay where I put it, because I didn't really have any good ideas for how to position its depth for welding.
Unfortunately, once it was in the hole, and once I moved it around any little bit at all, it already fit loosely enough that it wouldn't stay where I put it, it would just drop right through the hole. Part of the problem with that is that, the way the body is built, there is another panel just underneath the outer body, so there's no way to put a hand or a tool or anything on the standoff from the other side of the body panel. And if the standoff dropped in, it would be very difficult to get it out without cutting a hole in something.
Because of the inner body panel, the depth of the holes at the base of the rear window were just a little bit too shallow for the standoff to drop in. So those standoffs would need to be cut shorter, but for the time being, it allowed me to at least set the standoffs in the holes to see how they'd look. I decided that for aerodynamic purposes (by which I mean to say, "to look cool"), the best fastener to use would be a button head cap screw. I ordered these from McMaster-Carr, also.
Here's a picture of the two standoffs at the base of the rear window, with cap screws in them. Each one also has a washer on it in this picture, because even though I was pretty sure they couldn't drop in, I wanted to make sure they wouldn't.
So next, I cut the standoffs short. I had to cut the two at the base of the window so that they could fit, but I cut the upper ones, too, just because I wanted to minimize the dead space between the end of the cap screw and the bottom of the standoff. To cut them off, I just clamped each one in a bench vise and cut it with a cutoff wheel on an angle grinder. Crude, but effective. And the cut didn't need to look pretty, because it would be completely concealed after installation.
So, I did all those steps over the course of the week, spaced out over a few weeknights. The plan all along was for my buddy John to do the welding, because he has a welder, and (much more importantly) a good amount of practice using it. It looked like we were going to be ready for Saturday, so the plan was to go ahead and weld the standoffs in after breakfast. The weather, just for the record, was beautiful, so we rolled the garage door up and went to work. The picture below is looking up the street behind my house, at the end of my driveway.
To do the final preparation for welding, I used a sandpaper wheel on the Dremel to remove the paint around each hole, and I used the wire wheel on the bench grinder to remove the zinc plating around the surfaces of the standoff that would be welded. Then, John put each standoff in the bench vise and welded the end of each one shut.
Next, we went to place the first standoff in the body and weld it. My idea had been to make a small clip, shaped like the letter "Omega" from the Greek alphabet. The idea was that one of the cap screws would fit through the circle of the "Omega clip," and hold it to the stand off, and then the "legs" of the "Omega clip" would keep it from dropping through the hole. That would hold the standoff in place for John to tack weld a couple places, then the clip could be removed and the weld completed.
Well, it turned out that the whole arrangement was pretty clunky and didn't look like it would work very well. Fortunately, John had a much better idea, which was to use a magnet to hold one side of the standoff flush with the body while John tacked the other side. This sounded like a much better idea, so we went ahead to put the first standoff in place, and ... damned if it didn't drop right through the hole and into the small cavity inside the body.
I had almost kind of assumed that at some point we were going to lose one of the standoffs into the body, but I had figured that we'd mess around for a little while, and eventually fish it out of there. What I never would have imagined was how difficult it was just to find the standoff after it fell in. The space it fell into is so small, it seemed like it couldn't have gone far, but with extremely limited access into that space, it turned out to be seemingly impossible to even locate the standoff. We fished around with wire, magnets, and a mirror. We thought maybe it had bounced over and fallen out of one of the holes at the bottom of the cavity and fallen in to the trunk, so we emptied everything out of the trunk, and still didn't find it. We took turns laying on our backs in the trunk, fishing around and trying to find it. We must have looked for it for about an hour, and actually, we never knew where it was until finally I had just tried fishing a length of wire around in there again, and pulled it out, and then I bent down to look in there, and suddenly I saw the standoff just sitting there, in plain view, where I could just reach in and pick it up. Crazy.
So, then we decided to try the Omega clip. But, it turned out to be every bit as clunky and ineffective as it had looked like it would be. Fortunately, John had another better idea, which was to just thread a long bolt into the standoff, which would allow him to hold it where he wanted it with one hand while he tacked it in place with the other hand. He also left the bolt in place while he finished the weld, to keep slag and weld splatter from fouling the threads. That worked really well after a little practice on the first standoff, and from that point John was off and running, welding all four standoffs into the body.
After welding, John went around with a grinder and ground all the weld beads down.
Then he went back, welded over each one again, to try to make sure there was good weld penetration all the way around each one, then he went back with the grinder again. After that, each one looked something like the picture below.
The body shop should be able to clean up what's left, smooth out and/or fill in any imperfections, and it should be good to go. I'm a little bit worried that the body shop will present me with a list of all the reasons why this was a stupid idea and tell me I've ruined the car, but I really think it should be good to go.
The last thing I did was to wipe some used motor oil over the exposed sheet metal to prevent rust, and then put a cap screw in each one, just to see how it would look. The next thing will be to make the straps that will run across the window.
I'm not going to make the straps, though, until the car is back from the body shop. That's because I'm also a little worried that the body shop might want to, or need to, replace the panel at the base of the rear window, which would mean removing the panel with the two standoffs in it. If that's the case, we can always repeat this whole process again, but hopefully they'll be able to just repair any rust damage on that panel and use it as-is. We'll see how it goes.
Anyway, I did get some material to use for the straps. Lowe's had some strips of aluminum that are perfect for the job. They are six feet long, one inch wide, and a sixteenth of an inch thick. I should be able to cut these to length, bend them to match the contour of the window, and put holes in them for the cap screw to go through.
I think I will also try to put a thin gasket under each end of the strap, to protect the paint under it, and maybe some kind of weatherstrip-type padding under the strap, to cushion it from slapping against the rear window if airflow ever gets under it or anything like that.
Anyway, me and John had finished all this before lunch on Saturday morning, and I already felt like I'd accomplished so much, I basically goofed off the rest of the weekend.
Hopefully my transmission will show up this week, then maybe I can do the drivetrain test fit next weekend.
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