Jim Smart
September 14, 2007
When ordering date-coded glass, be very specific about where you want the "Car Lite" logo. Our side glass arrived with inconsistencies in logo positioning because we failed to specify where we wanted them.

Windshield/Backlight Installation
One of the biggest complaints we receive from readers is windshield and backlight leakage. In nearly 30 years of publishing, we've spoken with with all kinds of glass installation professionals and restorers. One common denominator holds true: None of them can guarantee your windshield or rear window won't leak. If you own a classic Mustang with a rubber gasket windshield or backlight, count on some kind of leakage. If it doesn't, you're fortunate; if it does, you're not alone.

For windshields and backlights to be leak-free, you need a perfect bond between rubber, sealer, and surface. This takes close examination of all surfaces before installation. Any irregularities must be corrected.

Your Mustang's roof and A-pillar seams aren't perfect. They were spot-welded together at a clip of about 60-75 units an hour at peak production. Each spot-weld depression is a potential leak-and there are dozens of them. Your objective is to minimize these irregularities. If you're doing bodywork and painting-or even replacing a windshield-fill spot-welds with body filler; then sand them smooth before priming and painting. Once you do that, the next obstacle is the reproduction rubber gasket and replacement glass. Neither is perfect. Replacement glass is thinner than original equipment. Reproduction gaskets, similar to original-equipment Ford gaskets, can be irregular, causing leaks. That's why proper use of the correct sealant is so important.

According to the Ford Shop Manual for windshield and back-light replacement, it's messy and frustrating. In 30 years of working with classic Mustangs, we've tried all kinds of approaches. Here's what we learned: Ford suggests sealant between the gasket and glass. This is a good idea because flexible sealant fills in irregularities-just don't use too much.

Between the body and gasket, Ford suggests sealer in the groove. If you do it this way, it gets very messy and doesn't always prevent leakage. Based on our experiences, it's best to install the glass and rubber gasket assembly without using sealer in the outer groove. Also, use soapy water as a gasket lubricant. This allows the gasket to seat smoothly before final sealing. Firmly lean on the glass across its face to seat the gasket.

This is a typical cutaway view of a gasket windshield installation. Install clips first, and put a dab of sealant at the base of each clip. Inject sealant into the inner groove before installing the gasket on the glass. Don't overdo it.

Leakage control comes from ensuring a solid wall of sealant between gasket and body. Once the gasket and glass are firmly seated, fill the area between the gasket and body with sealer. Do it in layers-down deep first-then allow it to set; next apply a second layer, also allowing time. Right before you install the window trim, lay down a top layer of sealant, leaving trim clips exposed. If you lay down too much sealant, it gets messy and the trim is hard to install.

How Is Glass Made?
Auto glass is made of sand, soda ash, coal, and other compounds that are mixed and fed into a furnace known as a glass tank. In the furnace, the compounds melt to become a molten mixture. The mix is then charged into a forming chamber known as a float bath. In this chamber, molten glass is cut off from the atmosphere, floating in a bath of molten tin (metal). During this process, glass is stretched to the thickness and width desired, then cooled until solid. It goes through a process of cooling and heating until it reaches the consistency desired. At this stage, the glass is a huge sheet about 700-feet long, ready for cutting and shaping. It's scored where necessary for cutting into a manageable size for automobiles.

Side and backlight pieces are made of tempered plate glass. It's very hard and explosive when broken, shattering into hundreds of pieces. Tempered glass is formed out of the glass-making process when it's molten and ready for forming. Edges are smoothed to make it safe. Holes are drilled where necessary.