Sealant for thermostat housing
 

I just replaced the thermostat on my 454 and used Permatex Hylomar to seal both sides of the standard paper gasket. It leaks (seeps coolant) all around. The water outlet (thermostat housing) and mating machined intake manifold surface were clean with little or no pitting. The water outlet was replaced about 3 years ago. I thought Hylomar was the "good stuff", but apparently the Permatex version, though expensive ($9 small tube) is not effective.
So, what is best to seal that gasket? Should I use the expensive thick Felpro gasket with built in O ring? If I used the Felpro gasket I will have a lot of cleaning to do first, and I am a little skeptical about getting a good seal on these old surfaces with just a thin O ring (Felpro says to install it dry), since there is some slight pitting in some areas, even though I did sand everything smooth prior to assembly.

Re: Sealant for thermostat housing
 

http://www.gasgacinch.com/index.htm Over the last 30 years, I've found this is the only sealer I use.

Re: Sealant for thermostat housing
 

ive always used permatex black or blue silicone... and in a pinch that form a gasket stuff that is like varnish

Re: Sealant for thermostat housing
 

If the mating surfaces are clean and smooth, then RTV shouldn't be needed.
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Re: Sealant for thermostat housing
 

I like aviation gasket sealer

Re: Sealant for thermostat housing
 

When you take it apart, run a flat file across both gasket surfaces to check for flatness and correct if necessary. You shouldn't really need a sealer in that location.

Re: Sealant for thermostat housing
 

permatex high tack. only thing i use on gaskets.

Re: Sealant for thermostat housing
 

Make sure the bolts you are using are not too long.

I chased a leaking thermostat housing gasket for a long time before I traced it to bolts that were just a bit too long, and although they contacted the thermostat housing when snugged down, they couldn't properly compress the gasket.

Re: Sealant for thermostat housing
 

I took the thin paper gasket from NAPA out, cleaned off the Hylomar and remnants of the gasket, globbed a lot of Hylomar (a bead all around the bolt holes and the flange), and used an Autozone gasket (thicker, light blue gasket, not paper) and bolted it down. Looks like an extreme amateur job, but so far it is not leaking. I managed to break off the eyelet on the ground wire from the A/C compressor which was mounted on the left stud, but fortunately I had one eyelet in my electrical toolbox to replace it.

Re: Sealant for thermostat housing
 

Just use the blue gasket. I think fel-pro makes it. Clean off the old stuff real good. Also make sure your filler neck gasket is good also. I just went through this mess myself.

Re: Sealant for thermostat housing
 

I used gobs of Hylomar on the blue gasket (see pic) and it seems to be holding OK now.

Re: Sealant for thermostat housing
 

I've always had better luck with a stock paper gasket on thermostats. I also have had to learn to be patient and allow the rtv to set up a bit before I torque the bolts down, and as my kids can attest to, patience is not one of my virtues.

Re: Sealant for thermostat housing
 

I don't like the mess gasket maker leaves on top of the intake. I always use the thick blue Fel-Pro and it's worked everytime I have tried it.