ElectroClassic EV
Classic Cars Reborn into the Electric Future

Glass Action

I deliberately delayed mounting the windshield because it would somewhat hinder the dashboard installation.  But now the dash is completely in, all wiring can be done from underneath, and there is no further need to delay.  My reference is the very thorough 914 windshield installation picture tutorial on the 914World forums, and what follows is my own comprehensive take on the process.  To start, I scuffed the edges of the window frame to promote the best adhesion possible, even though the butyl sealant I’ll use is one of the stickiest substances known to modern man.  It begs the question regarding the stickiest substance known to ancient man, but I digress.

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Butyl is actually a synthetic rubber that has many commercial applications, from sealants and additives to a food-grade variety still used in chewing gum.  Using a wad of gum to seal a leak is entirely justifiable on those grounds, and lends new respect to the fine art of  hillbilly engineering.  Butyl was long used to mount windshields in cars until they were expected to contribute greater structural integrity to the car frame, at which point urethane was adopted for it’s stronger adhesive property.  One pitfall of urethane sealant is that it squishes out thinner than butyl, and does not provide the windshield height that the original window trim expects.  Some installers use spacers for that reason, but I’ll stick with butyl and the original design.  My butyl ribbon was purchased from FinishMaster in the 5/16 inch wide version.

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Next, the 19 trim clips are inserted into the 19 existing holes around the outside channel of the windshield frame.  These clips hold the trim flush to the body around the circumference of the installed windshield, and are fastened into each hole with a built-in plastic rivet.  The “tooth” side of the clip points away from the pinch weld, and lays face-down in the windshield channel (see above).  I used a pair of smallish needle-nose pliers to straddle the center rivet sleeve of the clip from the top, and then pushed down so the nub underneath settled into the frame hole.  Once the clip is seated flush, the rivet pin can be drifted home with the flat surface of a small bolt or socket extender.  My first attempt to drift a pin with a punch destroyed it.  Luckily, I bought extras.

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Before going any further, a test fit of the window is crucial.  A couple rubber spacers serve as feet to position the glass properly during the install.  The generic spacers provided in the butyl ribbon box gave just the right thickness to center the windshield vertically.  Once the window is perfectly situated, there is a useful trick that will allow the replacement of the glass in the same position every time.  Start by placing a piece of masking tape across each seam where the window meets the frame on all four sides of the glass, and draw a line down the center of the tape for additional reference.  Leaving the tape in place, use a razor blade to cut it along the edge of the glass so the windshield can be removed for the remaining prep work.  The masking tape will now serve as a positioning guide during the final placement of the glass.  Before removing the glass for final cleaning and priming, I suggest taking a close look at the overlap of the windshield edges over the pinchweld frame all around the circumference.  It’s like taking a few test laps to familiarize yourself with the track before committing to the race.

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The next step is to thoroughly wipe the edges of the glass and the window frame with cleaner and a dust-free cloth to remove any residual contaminants.  From this point forward, it’s important to wear latex or nitrile gloves to avoid fouling these surfaces with skin oils. Common wisdom then says to prime both glass edge and window frame for better adhesion with a product called Pinchweld Primer for Butyl Tape.  Some might say butyl is already plenty sticky, but given the option of a stronger bond, I’ll take it.  I applied primer to both the glass and the frame with the brush included on the bottle cap, and then let it set up for about 15 minutes.  By the way, I wasn’t too impressed with the free brush.  Wild bristles continued to sprout as I worked, creating messier edges than I wanted.  So I switched to a better brush.

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In the image above, you’ll notice how the window tinting was designed to allow a margin of raw glass at the edge for proper adhesion.  If there was tint film between the butyl and glass, that film would be the only thing holding the windshield to the frame.  This untinted margin will be painted completely over with primer all the way around, resulting in a stronger seal and also masking any yellow body color that might show through the glass.

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The next step is applying the butyl ribbon to the window pinchweld frame, in preparation to lay in the glass.  It’s possible to stick the butyl ribbon to the windshield first, but I wanted more control over where the ribbon adhered to the metal frame.  I unrolled the ribbon from the spool as I went around the frame, pressing the raw butyl to the black primer and leaving the top protective paper strip attached.  Coming completely full circle around the frame, the ribbon must then be joined to itself.  Some installers favor the overlap, some promote the side-by-side method, and others recommend the butt-splice (including the instructions on the box).  All methods except the last leave an excess of butyl at the joint, creating an inconsistent thickness that could compromise the seal.  Although the butt-splice requires more skill, it’s perfectly effective if there is no gap in the join.  I made my splice at the bottom edge in front of the passenger washer nozzle, as seen above.

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I’ve never worked with butyl before, and expected a moist and messy kind of sticky, but instead found a clean and dry kind of sticky.  It’s possible to directly contact the butyl if you don’t give it time to bond to your fingers, like handling salt water taffy with a quick, light touch.  I also found that normal scissors will easily cut the ribbon without sticking, as long as it’s snipped quickly and without hesitation.  Once the butyl was attached to the pinchweld frame, it was easy to mold by lightly pushing it around like putty – especially handy for the butt-splice.

During the earlier test fitting, I had a close look at the edges where the glass overlaps the pinchweld lip, and noticed that they mostly were not flush.  In other words, even though the pinchweld edge was touching the glass on the inside, its surface angle dropped away from the glass toward the edge, leaving an overhang space under the outermost circumference of the windshield glass.  I assumed that void was meant be filled with butyl, so before going any further I carefully molded the ribbon away from the channel and towards the edge of the pinchweld surface.  Subsequently, when the windshield is finally lowered into place, there will be maximum butyl-smoosh between glass and frame.

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Time to lay the glass into the frame.  Following the 914World tutorial, I originally decided to lower in the windshield from above while standing inside the cockpit.  I didn’t lay the rubber spacers into the channel, concerned they would touch the butyl and create a mess.  Putting the spacers on the ledge above the channel seemed like the next best idea, but the windshield wouldn’t balance there in a trustworthy way.  It occurred that I had a better shot floating the windshield in from beside the front passenger fender.  I was able to balance the bottom center of the glass on my right hand fingers while aligning the masking tape marks.  Once things were lined up, I carefully tilted the glass into the frame, letting it adhere to the ribbon.

There’s no turning back once the butyl touches the primed glass, and the only way to undo it involves removing the windshield, cleaning off the butyl, and starting completely from scratch. (Fortunately, there will be no blog entry on that topic.)  Before pressing the glass into the frame, there’s still time to adjust it on the butyl to a very minor degree, levering the window edge with a wooden implement to avoid chipping the glass or paint.  Of course, this should be done before finally compressing the glass carefully against the butyl around the window edges with an open palm, being careful not to stress the glass.

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Mounting the rear window is basically a smaller version of the windshield install.  The targa bar interior trim should not be mounted yet, as it obstructs access to the rear window frame.  Prepping the frame and window glass is exactly the same, including the use of rubber spacers and masking tape to properly align the window before laying it in place.  The only difference is that the rear window lays in from inside the car rather than outside, and it needs to slide from the firewall upwards into position before seating on the butyl.

I actually installed the rear window first to get some practice for the windshield, and it was a piece of cake.  However, my one-man windshield operation involved equal parts of stress and luck, and I would definitely tag it as a two-man job for future reference.

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2 Responses to “Glass Action”

  1. If I haven’t said it before, or quite often enough, you’re absolutely amazing, and I’m so proud and glad to have you in my life. Rock on!!!!


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