I replaced my 180F thermostat with a 195F one before taking my emissions test (which I passed in spite of having no EGR or AIR system). I'm now getting slight weping of coolant from the thermostat housing flange, even though I redid it a second time using silicone gasket goo. It's a paper gasket, but I've seen c-shaped rubber o-ring ones on Cavaliers, that fit around the thermostat, and ought to seal better. Did all 3rd gen small blocks use the paper gasket, or did they later go to a rubber ring? It almost looks like one might fit.

torque it correctly, i just use RTV - no leaks

Go to the dealer and get an OE gasket. It comes with an impregnated seal as opposed to being straight gasket material. You can forget the RTV with it and it's a "once&done" deal. The shaped rubber gaskets you refer to are used in later GM vehicles that use an entirely different thermostat.

I make sure all surfaces are clean and dry, then use a paper gasket lightly coated on both sides with form-a-gasket.

i use rtv sealant with a real thick gasket compared to the paper size.. works great..

I've been known to not have a spare gasket around I've used the same one before, course I had one of those real thick rubber types. Like Justins86bird said make sure you get all the surfaces clean (gaskets are made to work on clean even surfaces), make sure you get all the bolts tight again too, and make sure the thermostat is in the seat right too... might sound stupid but I've seen people crack and even break ears off the housing from not having it totally in correctly.

Thanks for all the suggestions, folks! I've been changing thermostats for dozens of years and hardly ever had one leak that I can remember. It's only recently that I came across one of those rubber rings and wondered if late 3rd gens used them. I think my problem here was that I originally tried to use the paper gasket with no sealer, and then, after it leaked, took it apart and coated it with RTV. The gasket was already soaked, so maybe some coolant wicked through it. The leak was extremely minor and appears to have disappeared now, so I'll leave well enough alone before I strip a thread in the intake manifold.

goto ur parts store and pick up a reusable gasket. it comes in thick rubber (blue colored i think). u can reuse it as many times as u want and it won't ever die on ya. plus no cleaning up mess whenever u change ur thermostat. iv replaced mine thermostat 3 times so far, and with a reusable gasket, it makes life so much easier. only cost about $4 too

Maybe next time. I ended up getting the GM gasket from the dealer (just 2 northern pesos) which has a ring of silicone on both sides. It sealed with no problem. All I did was clean off the surfaces with Scotchbrite.

(Unfortunately, I stripped one of the bolt holes in the intake manifold with all the back and forth, so I had to tap it out to 7/16-14 )

i replaced mine and used a 99 cent gasket from pepboys and it works fine. i used some rvt and it works great and no leaks