Sealant for thermostat housing - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tucsonjwt · 07-31-2011, 07:18 PM

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.

VegasBruce · 07-31-2011, 07:31 PM

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

**old Rusty C10** · 07-31-2011, 09:21 PM

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

67_SS396 · 07-31-2011, 11:14 PM

If the mating surfaces are clean and smooth, then RTV shouldn't be needed.
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88Jimmy4x4 · 08-01-2011, 07:49 AM

I like aviation gasket sealer

njlimbaugh · 08-01-2011, 10:35 AM

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.

thesickc10 · 08-01-2011, 01:29 PM

permatex high tack. only thing i use on gaskets.

raceman6135 · 08-01-2011, 02:00 PM

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.

tucsonjwt · 08-01-2011, 03:33 PM

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.

TIM86 · 08-01-2011, 10:11 PM

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.

tucsonjwt · 08-01-2011, 10:53 PM

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

jimbonice · 08-01-2011, 11:33 PM

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.

dropd80s · 08-02-2011, 09:31 AM

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.

07-31-2011, 07:18 PM	#1
tucsonjwt Registered User Join Date: Jul 2007 Location: Tucson, AZ Posts: 3,191	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.

07-31-2011, 07:31 PM	#2
VegasBruce Registered User Join Date: Apr 2011 Location: Just outside Sin City Posts: 239	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. __________________ Bruce 73 Stepside, 74 Blazer, 75 1 Ton, 66 Barracuda and a 05 Electraglide DD http://i56.photobucket.com/albums/g1...Rusti192-1.jpg

07-31-2011, 09:21 PM	#3
old Rusty C10 Robert Olson Transport Join Date: Oct 2002 Location: recent transplant to NC USA Posts: 20,316	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 __________________ Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!!

07-31-2011, 11:14 PM	#4
67_SS396 Registered User Join Date: Mar 2000 Location: Blue Ridge, Tx Posts: 1,172	Re: Sealant for thermostat housing If the mating surfaces are clean and smooth, then RTV shouldn't be needed. Posted via Mobile Device __________________ Fawker Fab three seater rock crawler buggy 498 BBC/SM465/203/205 doubler, propane, 5.13's, 42" PB Rockers '66 C10 - The new hot rod '77 3+3 Dually - 12 valve 4wd conversion tow rig project '28 Ford 2 door sedan street rod RevLimit Offroad "Hang on beer, we can do this."

08-01-2011, 07:49 AM	#5
88Jimmy4x4 Registered User Join Date: Oct 2003 Location: houston, texas Posts: 1,974	Re: Sealant for thermostat housing I like aviation gasket sealer __________________ 2008 Silverado Z71 RCSB 1991 GMC Jimmy 4x4 1976 GMC Sierra C15 SWB

08-01-2011, 10:35 AM	#6
njlimbaugh Registered User Join Date: Jun 2011 Location: LaFayette GA Posts: 111	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.

08-01-2011, 01:29 PM	#7
thesickc10 Registered User Join Date: Sep 2010 Location: New Hope, PA Posts: 279	Re: Sealant for thermostat housing permatex high tack. only thing i use on gaskets. __________________ 1985 c10 355 th350 (working on 4 speed conversion to super t10) 1979 camaro 358 super t10 2003 f250 superchip slp exhaust

08-01-2011, 02:00 PM	#8
raceman6135 Dino burner Join Date: Nov 2002 Location: Calgary, Alberta, Canada Posts: 602	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.

08-01-2011, 03:33 PM	#9
tucsonjwt Registered User Join Date: Jul 2007 Location: Tucson, AZ Posts: 3,191	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. Attached Images

08-01-2011, 10:11 PM	#10
TIM86 Registered User Join Date: May 2011 Location: St.Petersburg,FL Posts: 158	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.

08-01-2011, 10:53 PM	#11
tucsonjwt Registered User Join Date: Jul 2007 Location: Tucson, AZ Posts: 3,191	Re: Sealant for thermostat housing I used gobs of Hylomar on the blue gasket (see pic) and it seems to be holding OK now.

08-01-2011, 11:33 PM	#12
jimbonice Registered User Join Date: Aug 2010 Location: White City, SK Posts: 411	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.

08-02-2011, 09:31 AM	#13
dropd80s Registered User Join Date: Mar 2003 Location: wichita falls tx Posts: 429	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. __________________ "I'm Dirk Diggler enough to own and operate a vehicle of this caliber" -veinbar Dropped and LS Swapped SWB 87 Build Thread



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