I have an 87 745 with a cracked windshield, and two good used windshields to choose from, both of which are already out of the car and cleaned of old sealant. As I was the one who removed them from parts cars, I think I can handle the procedure of installing one of them in my car (getting the old one out, is obviously, not a problem) but I have a couple of questions, primarily concerning the adhesive to use. I have read that the original material is butyl, and have obtained some 3M Butyl tape, but then I read the FAQ section of this site in which someone (working on a 240, incidentally) recommends using urethane instead. My concern with this is that it might make it very difficult to remove the glass should it become necessary to replace it again. If the butyl tape is suitable, should all the old material be removed from the pinch weld area in which the glass seats, or will the new adhere to the old satisfactorily. Is any sort of primer or activator needed, or just put the glass in place and gently, gradually press it down? What about temeperature? I imagine it would work better in warmer conditions.

I would appreciate any advice from anyone who has done a 7 series windshield.

Thanks in advance

I have done many,I feel the best adhesive to use is is primerless quick cure urethane,it comes in a tube style cartridge dispensed with a caulking gun,if you use the short cut method which is favored by most autoglass installers then the idea is to cut the w/s out leaving as much butyl on the pinchweld as possible.next you will lay a 1/4" bead of urethane all the way around across the top of the butyl and then lay on the w/s pressing firmly around the perimeter(there should be two rubber setting blocks along the bottom of the frame aprox 4" inward from each side,these will keep the w/s from sliding down while waiting for the sealant to dry,if you have no setting blocks then you will need to keep the w/s in place with blue painters tape.ureathane is also great for holding the trim on.
If you want to use butyl it works ok but you need to remove all the old butyl and yes you do need a primer made for butyl it comes in a can with a brush applicator,over lap the new butyl at the bottom and do not touch the contact surface of the butyl because that can cause leaks.
just my two cents,good luck

Thanks for the information. It looks like I will need to either get the butyl primer or go with the urethane.

If I use the butyl, and clean off the pinch weld completely, will a single (1/4", I think) bead of it be sufficient? Overlap it at the bottom? By how much?

The setting blocks on a 740 are not made of rubber. They are rather complex plastic devices that mount in the same way as the molding clips, but consist of two pieces that have ratcheting teeth on them so that they can be adjusted to the correct height. I have a supply of these that I have saved from cars I've dismantled.

I don't expect to have to glue the moldings on, as I've usually been able to get them off without breaking the clips, and if I do break some, I have plenty of spares of those as well.

the butyl that is used on the 740 is 5/16 round or square (it comes on a roll)
Do not mash it down prior to setting the w/s except where you over lap the joint at bottom and then only enough to make the height of the seal consistent.
When using this method cleanliness is everything if you don't want leaks.

The butyl tape I have is a 3M product, round, probably 5/16" diameter.

Today I got a can of Betaprime Pinchweld and Encapsulation Primer. It says on the can that it's for urethane but the guy in the shop insisted that it was what they use with butyl tape.

How do I use this product? The can is covered with warnings in three languages, but nothing in the way of directions.

I would assume, first remove all of the old butyl from the pinch weld area and clean it thoroughly, probably with lacquer thinner, and allow to dry completely. Then apply the primer, but do I allow it to dry, or work quickly and lay the tape in place while it's still wet? Once the tape is in place, do I apply primer to the glass, where it will contact the tape, to the tape itself, or not at all at the glass/tape contact area? And if primer is needed here, do I wait for it to dry or set the glass while it is still wet? Obviously, the glass will need to be thoroughly clean as well.

After all the glass is in place, go around and around, carefully pushing down, to compress the tape and (hopefully) form a good seal without cracking the windshield, then reinstall the moldings.

Betaprime will work fine,
clean the pinch weld,
prime pinch weld and allow to dry,
do not prime glass just clean and dry well(keep in mind new razor blades will not scratch glass)
do not run the butyl too close to inside edge of pinchweld or you may end up with sticky edges inside the car when you are done,
as extra security against leaks you may wish to apply autoglass silicone around the perimeter of the glass after setting it and then tool it up to the glass edge,
if there are any drain holes along the bottom of pinchweld do not fill them in,
if you choose to use silicone make sure that it is for autoglass use.A lot of silicone products have an acid base to help with adhesion,over time these non autoglass silicones will remove the paint from the surface causing rust and leaks.

you should not have to press very hard on the w/s to get it seated,kind of work around it with the flat of your palms about 8" apart from each other and not too much pressure in any one point,time,temperature,and gravity will finish the seal for you.
good luck

Great tips: I put these in the FAQ.

Steve,

I saved a copy for myself, and was thinking it would be a good addition to the FAQs.

I suspect I have a leaking windshield and was thankful to get the tip on using autoglass silicone.

Charley

I have been an autoglass technician for 20 yrs.and my honest opinion is that urethane is far superior to silicone or butyl.