Thermostat gasket leak - best fix?

Reprise

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Well, as it turns out, my coolant leak at the lower hose where it joins the t-stat outlet - is actually coming from the bottom of the thermostat gasket. Was able to get a mirror on it and saw the telltale wetness on the thin edge of the gasket; verified it by feel.

Was solid for almost a year. Must've given way when I stressed the motor during my recent trip, running at +3000 RPM all day, as it never lost a drop of coolant until then (but loses it on every drive since then; I found the leak nearly immediately using a pressure tester two days ago)

So...I need to redo the gasket. Searched online & this site for gasket vs. RTV, etc. Nothing here on this specific issue, especially for the V8.

The one that's in the truck now came from Gates, along with the water pump. Or from Stant, as I had to purchase the 'right' (early model, with 'integrated housing') 5.3 t-stat when I did the WP last year. I forget 'whose' gasket I used, but it was one of the two.

(note: Stant's site doesn't show or mention the gasket; I know one came in the Gates kit, so I must've used that. Their site let me pick the 5.3L, then showed me only the 4.2L water pumps) :Banghead:

As an aside, the t-stat itself is just fine. I have no overheat / underheat issues at all. Just the loss of coolant; I was lucky I checked the level in the overflow bottle during my trip & since then.

Also, when I installed it last year, the bolts were torqued to the proper spec, per the manual (I think it's 17 ft/lb, just looking online tonight, but I know I did it to spec, whatever it is)

Options:
- Another gasket just like the 1st (paper, I think)
- Some other 'improved' gasket (I saw Fel-Pro rubber mentioned - ?)
- Regular gasket with gray RTV; let it set up overnight, then torque it down (found this on a pickup site, concerning a 5.3L truck)
- Skip the gasket altogether and RTV it up.
- Some other option I didn't mention.


What say the other experienced Gen III / 5.3L owners? (and if you had a t-stat leak, you go to front of the line w/ my 'thanks' for your answer)
 

mrrsm

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The "Baker's Dozen" for all Paper Gasket R&Rs:

(1) Clean all surfaces religiously spotless with Acetone or Lacquer Thinner... even the NEW Thermostat surfaces.
(2) Use the Factory or OEM Quality (Felpro) Paper Gasket.
(3) Coat the gasket contact margins of the Thermostat Flange with an even, thin layer of Permatex High Tack Gasket Sealer. Let it sit for 10 Minutes.
(4) Coat only one (1) side of the Paper Gasket with the same stuff ...and allow it 10 Minutes to "Tack" up.
(5) Carefully align the Thermostat Manifold with the coated side of the new Paper Gasket and once accurately aligned... press them evenly together.
(6) Coat the exposed contact surface of the Thermostat Gasket and allow it to sit for 10 Minutes.
(7) Clean & razor scrape surfaces of the Engine Head-Block at the T-Stat Port and apply an even, thin coat of the Permatex High Tack Gasket Sealer.
(8) After the 10 Minute prep time... carefully align the T-Stat with the Port and as soon as they make contact... Thread the Fasteners in.
(9) Tighten the Fasteners according to the OEM Installation instructions.
(10) Don't wear your favorite shirt or good clothing or fail to use Nitrile Gloves, If you get this stuff on fabrics and it dries... it will NEVER come off,
(11) If you get this highly adhesive stuff on your fingers... You'll almost have to wait for new skin to grow in before it will loosen and come off.
(12) Keep the Permatex Combo Brush Applicator Lid screwed down tight inside of the can... or the stuff will dry out faster than a Popcorn F*rt.
(14) If done properly... T-Stat Gasket(s) will never leak again. (...and yes... I skipped a certain number ...on purpose... [It is Very Bad Luck]....)

You can pick up the Permatex High Tack Gasket Sealer from this place and other locations:

https://www.autozone.com/sealants-g...h-top-can-high-tack-gasket-sealant/554063_0_0
 
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Mounce

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Mar 29, 2014
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Tuscaloosa, AL
Same water pump/thermostat as the trucks? I know my 03 Silverado had a rubber O-ring for the thermostat and a corresponding groove. No paper.

Just go to your parts store and ask for a thermostat gasket, they usually have them for most vehicles. That or buy a new thermostat which will come with the gasket.
 
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Reprise

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Oh yeah - I know the gaskets are available solo - I remember the times I'd get one at the parts counters for my old SBCs, back in the day. I guess my question was around whether or not to use one, 'supplement' it, or just skip the gasket entirely & use RTV instead.

I know it also has the O-ring type, and I did use a new one there.

@MRRSM - appreciate the step-by-step and your always thorough replies. I will follow them to the letter...er...number...lol.

On edit: Uh-oh... *only* the o-ring gasket s/b used? Will validate that when I get back from my tonneau trip this morning. thx @m.mcmillen !
 
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