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.