Hey,
First I have searched thoroughly here and found some great information, which I have put to use, but I am still having no luck.

The problem is with installation of a new shower screen and 8.5mm gasket into my second-hand isomac millenium. The machine has undergone a bunch of work recently, I had to extract the old carbonized gasket by screwing a drywall screw into it, then cleaned up the group head and brass dispersion screw thing with a scotchbrite pad. Purchased the new shower screen and gasket locally at visions espresso. The gasket says Faema on it. I've tried installation 5-6 times and I can't get the gasket to seat evenly, one side always pops out, even after twisting a portafilter with basket all the way in. i've tried it both with the machine cool and hot. In my last attempt, I put the gasket in my freezer for 4hrs, had the machine fully warmed up, lubed the gasket with foodsafe grease, used a PF w/o basket to seat it, then another twist with a double basket. The gasket popped out again, so I spent 15-20 minutes poking it into place with the rubber coated end of a plier handle, then again with the PF & double basket. It is still popping out. I have worked it until the popped-out spot did a full 360-degree rotation, it still will not seat.

Any other tips on this?
Seems like I will be buying another gasket, just to rule out that as the culprit. Should I seek out a different brand or thickness?
Do I need the paper washer for successful installation?
Is there a heating/cooling trick I can do to help lock it in? (beyond chilling the gasket in a freezer for several hours)
Or maybe just "try a different gasket"?

Help please!
I do appreciate it!

try another gasket

make sure there is no grease in gasket groove.

paper shims are just to correct for worn pf; to get them to centre when locked.

The 8.5mm PF gasket I got from Stefano's Espresso Care in Oregon worked perfectly in my 58mm E-61 Vibiemme. I would guess that the gasket you got is not the right one or it has a problem. This popping out thing shouldn't be happening. Either it is too big, or.. and this is a stretch... air is getting trapped behind the gasket and expanding and forcing the gasket out.

Does the old screen fit with the new gasket?

Is the new shower screen identical diameter to old?

New shower screen is slightly different than the original, but looks like a good fit.
The gasket fits... I can twist the PF fully in. But as soon as I remove the PF, one side of the gasket pops out. It's the same thing if I manually poke it in.

I wonder if it's the lube- either way I'm off to pick up a few more gaskets. Thanks for the sanity check!

FWIW, never used lube. First install of a new gasket is always a tad fiddly, but once in, it remains in place.

Always check a new gasket to make sure it's not some ossified relic found behind the parts bin. It should be very pliable. When squeezed to form an 8, there should be no cracks around the edge and it should spring back to a circle immediately when released.

I am assuming you are, but you are installing it with the tapered end up right. There is a top and bottom although if you put it in upside down, I doubt it would go in at all.

Sure enough, they had received a batch of incorrect gaskets from their distributor. I wasn't the first to have issues... kudos to visions- they were very cool about it. Replaced the gasket N/C and gave me a discount for the inconvenience on some other items. It took all of 10 seconds to install the correct gasket w/o lube.

Thanks again for the help!