Windshield/rear window. I'm undecided what to use. There are good reasons for both.:confused:

Urethane is a lot stronger and will not weaken with heat.
Both seal well, and are about as easy to replace a screen.
Butyl is more accurate for a resto, but who's gonna know?

Call a windscreen shop and get their opinion.
The guy who just replaced the screen in my Holden said the urethane is the better choice and why it's what the automakers use today.

The last windshield I had installed I was told I could get it done with butyl but it was not legal here in CA. The previous glass was done with urethane and the edge of the glass looked bad where the eurthane squeezed out as the glass was set in place. The glass guy said he could keep that from happening so I went with urethane again. The results were great, he used a neoprene tape as a barrier that gave a nice finished visible edge and it kept the urethane in place.

The factory never used butyl to install windshields or backglass. This being said and can be substantiated by John Z. from CRG.
Quote:"The factory used a "dam" and Thiokol pumpable adhesive with different silane primers on the glass and on the body opening flanges, which changed to urethane pumpable adhesive in the 70's. GM never used butyl in production, as it won't pass the MVSS 208 Frontal Impact Windshield Retention requirement, and the Thiokol or Urethane adhesive design makes the windshield and backlite structural parts, improving body torsional stiffness. Some glass shops use butyl for replacement glass because it's quick and cheap, but it's technically illegal. The factory rubber spacer blocks established the windshield height for correct flush fit of the reveal moldings to the glass surface."

The last windshield I had installed I was told I could get it done with butyl but it was not legal here in CA. The previous glass was done with urethane and the edge of the glass looked bad where the eurthane squeezed out as the glass was set in place. The glass guy said he could keep that from happening so I went with urethane again. The results were great, he used a neoprene tape as a barrier that gave a nice finished visible edge and it kept the urethane in place.
Thanks Dennis,
I feel better with urethane. I will tell the installer about the neoprene tape.:thumbsup: