thinking of using 3m window adhesive instead of butyl tape.has anyone tried this.

I haven't tried it. I'm comfortable with the fact that butyl is the factory spec, it's easy to work with, and it holds up over time. What benefits or upgrades are you expecting from '3m window adhesive'?

I presume you are referring to 3m urethane. Much trickier to use than butyl for a rear window with no real up side. Urethane on a vertical window requires rubber blocks in the bead, and mechanical clamping to hold the glass until it cures. If the bead gets too thin, you risk having the glass blow on a good bump. On the other hand, the back glass has no structural load, so you don't lose anything with the less rigid seal that butyl gives you, but get a much easier install in the deal.

it comes in a caulking tube so it would be easy to apply.it is inexpensive $12.50. for one cartridge.also i ask what size butyl tape ,and i get many different answers like: round, rectangle ,square, 1/4 inch, 3/8 inch ...........?do you know the correct size/ dimension butyl tape?.i have spoken to to a few auto glass installers, and they said the new adhesive is the way to go,but it is a 914 so they could be mistaken

ok thanks. i thought it might be easy to apply,and stick the glass to it like butyl.i will have to fully investigate.by the way what size butyl do you recommend.

Standard rule is 3/8" for bare metal install, 5/16" if layering on top of old, but stable and solid base left over from a clean hot knife removal. So, in most cases it will be 3/8". Square or round never made much difference to me. Once you set the glass and squish it down, they are both the same shape in the end.

Direct from the factory manuals. Trust the factory manuals, not glass guys who've never picked up a technical manual in their life.

8mm = 0.31496063" ~ 5/16" Butyl

If you think urethane is easier to work with, you haven't used both and been able to compare. Urethane is like trying work work with toothpaste, and butyl is like trying to work with sticky licorice rope. Butyl is far, far easier - just remember that there's no sliding the glass around. You get one shot and once it's stuck it's stuck.

On a small vertical window like a 914 back glass, 5/16" even on a "down to metal" install. You just need to be careful on the corners not to stretch it thin in the corners. Patience and a little back pressure as you lay the corner, it'll be all good.

A glass shop would use 3/8" tape 9 out of 10 times just out of habit on a clean metal install. As I climb into the Wayback machine, it seems to me I did often use thinner tape on nearly vertical installations (i.e. pickup truck back windows) or they would sit up too high.

1 beer for McMark. 5/16" is the right call.

ok fellas i'm going with 5/16 butyl $6.00 from local glass shop.thanks you for your enthusiastic support!

You can actually stretch the 3/8" to make it fit (just a technique). That's why the glass guys don't worry about the thickness, they make it work by stretching.