Engine/passenger compartment glass separator cracked

I got back home yesterday afternoon after showing the car at a local concourse, and put the car in the garage, checked around and everything looked fine. 3 hours later my son comes home and asks me how I got the crack in my window. We go out to the garage and sure enough there is a large star pattern crack in the glass separating the engine from the passenger compartment, directly behind the top of the driver’s seat. The center of the crack is high up near the roof, and a star pattern radiates out sideways and down to the bottom of the glass. Feeling around for a chip from some unknown source on both sides of the glass yielded nothing. Each side was perfectly smooth, and the crack is internal.

One very cold snowy winter day about 25 years ago as I was driving to work, I put the defroster on high to get rid of the remaining ice on the windshield. About 2 minutes later I noticed a small crack at the base of the windshield which rapidly grew into a large crack with starring, and then proceeded to the passenger side. There was nothing to feel internally or externally. No chip, no rock dent, just smooth glass on both sides. The crack was internal, caused by severe cold on one side and 80 degree heat on the other. I later found out from the repair people that this was not uncommon.

My car was in the sun for about 7 hours yesterday, then I took it for a spin before going home, adding engine heat to one side of the glass. Then I put it in the garage which is much cooler than outside. The crack is just above where my external/master ignition cutoff is mounted, and when shutting off the switch I didn’t notice anything unusual. As I could find no evidence of anything striking the glass (how could a pebble get in there anyway? It’s sealed off from the outside world by the engine cover on one side, the passenger compartment on the other side, and a full underbody panel from below) I suspect the damage was the result of thermal action. Maybe there was already an unknown weak spot in the glass. I don’t yet know if this glass is a typical tempered glass sandwich, or whether there are 3 separate panes with air space between them (it’s pretty thick). In any event, we are going to replace it with a coated Lexan piece instead. Same stuff that’s in fighter plane canopies, with the same military spec coating so it won’t haze or yellow or scratch, and has better thermal properties than glass.

I’m certain that the engineers who designed the part and specified the materials knew it would be constantly subject to extreme heat on one side, and took this into consideration. Trapped air is an excellent insulator, so perhaps we have tempered glass plates on the outside and some type of plastic/polycarbonate in the middle, but not all bonded together. Possibly air spaces on each side of the middle piece to keep the heat out of the passenger compartment. If anyone actually knows about the construction of this glass piece, or any cons to replacing it with Lexan please let me know.

So… It actually runs and goes and drives? That’s excellent news!

Unfortunately, I have zero information about that glass, what could’ve made it break, or how to do anything with Lexan other than the single sheets that I worked with before.

If it is multi layered glass (for noise insulation usually two or more glass layers of different thickness (so they don’t resonate at the same frequencies) plus air in between) it is very likely that the lexan will be much louder. You may be able to count the layers by shining a light through at an angle. Or just smash it up now that it is broken. I wouldn’t worry about the heat so much and there are infrared filters that can be glued on if it turns out to be a problem.

Glass does break sometimes if the cold AC hits it when they stood in the sun, and your heat change could have caused it. Maybe you can post some pictures of the glass.

Has this window problem been encountered on other cars? If so, what solutions and results?

A switch of materials as considered may solve the cracking issue. David mentions other issues worth considering when changing materials.

If IR filters are considered, be aware that IR, “cool”, filters operating at temperatures above room temperature have heat conduction and blackbody radiation heat transfer through the filter which is separate from the IR transmission coefficient measurements. A quick way to understand this is to feel the heat from a fireplace through a closed glass fireplace door. When the fireplace is just lit up and glass door closed, one sees the fire but feels little heat when near the glass. After a few minutes the glass is heated enough to now feel the increased heat when near the glass. The glass fireplace door has acted as an IR filter but once warmed has not reduced the heat transmission as much to the room as initially when cold.

Good information. I’ll need to do some more research to determine the best replacement material, but military grade Lexan with the coatings they use on fighter plane canopies would work well, as would Gorilla Glass Vectus, now available in sheets for windshields. Thanks for the explanation on IR filters, I don’t know much about that area.

I believe this was a combination of unfortunate circumstances. First, the service and parts manuals show that this glass bulkhead separator is supposed to be mounted to the bulkhead with plastic anchors and plastic rivets (scrivets). These would allow some small movement of the glass on expansion or contraction. However, my glass was mounted rigidly with metal bolts and nuts, which would not allow for much movement at all. Second, the heat factors mentioned earlier. Third, while the roads around here are excellent and well maintained (thank you to Westchester county property tax), the horse farm where the concourse was held only has dirt roads and they are not by any means smooth. Fine for horses, not so good for super stiff cars, so a lot of vibration transmitted through a very rigid suspension and body.

I think all these factors played a part, particularly the rigid metal bolts used instead of the softer plastic scrivets. In addition, I had noticed from day one that the internal rubber gasket around the area of the crack had come loose from the surrounding metal and was bunched up inside the glass where I could not get at it. So maybe something had happened in that area previously.

Anyway, the car ran well, the alternator has begun to work (sort of), and the gauges work some of the time (most likely a bad ground somewhere), so it’s going back to the shop to get sorted. But the few brief drives I have had convince me that this car is something special, or will be once it’s sorted out.

Thanks for the help guys.

1 Like

Here is a pic I took. Looks like a classic stone chip, with an epicenter, but it’s all internal. So at least 3 layers, glass on each side with a middle layer that can crack like that (maybe more glass with air spaces).

I can’t smash it yet, as I need the piece as a pattern for a new one. Can’t buy these anywhere around here.

That’s an interesting pattern. Doesn’t look as if it is toughened from how the cracks radiate out but not completely (I‘m definitely not an expert). There are glass thickness measuring devices that show how many layers there are and how thick they are. Just a laser at an angle reflecting onto a scale.
Could just be a three layer laminated glass and the middle one failed. The mounting might have played a part, or just the stresses while cooling down. I had a phone screen crack after I dropped it, but it was fine when I picked it up. Some minutes later it snapped. Hot day, cool pocket.
…good point, don’t smash it (yet).

Not available at NAPA?


1 Like

Ah Wiggles. I would have given up long ago if not for the laughs your comments bring on. Good to remember at the end of the day that this is still just a car, that problems are there to be overcome (lord knows we’ve done enough of that over 2 years), that the world has far larger problems than repairing an old car, no matter how valuable or interesting, and at the end of the day it’s the friendships you develop through this hobby that make it all worthwhile.

1 Like

Truer words, my friend!

With luck, someday I’d like to mooch a ride in th’ beast, so I can scratch that one off my list!

It’s been an amazing learning process for me, just reading the thread.

Next time you are in my neck of the woods we go to Lime Rock for some laps. As long as it’s not snowing:)

1 Like

We’ll save that for High Plains…:cold_face:

1 Like

You getting any yet Paul?

…snow? :crazy_face:

Above 10,000’, yea, it’s been dusted.

Nothing out on the high chaparral yet.