i've had better luck with liquid pipe sealant over rtv, seems the rtv always gets wipe off by the block threads before the stub makes it thru the block. last time i used the pipe sealant, i could look thru the water pump passage and see the threads still covered well with pipe sealant.
GM's thick ass black gasket sealer works great also, and seems to stay on the end of the studs pretty decend, so the water doesn't erode the stud so badly, but I believe it's been discontinued. I use this stuff now, and it's always still on the end of the stud when I take them out http://www.herchem.com/specs/grrip.pdf
I used Permatex teflon thread sealant once and had leaks at multiple studs within 2 days. Never again.
Toyota sells a very good sealer called FIPG...used to use when I worked at the dealer. Better than anything I've ever used that said permatex on the tube. It's expensive, but toyota uses this stuff instead of gaskets and their motors don't leak for 150K. I use this stuff on everything...its good for oil and water, I've never had a leak with this stuff.
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