When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Just fired up the 455 after a rebuild. Was great to hear it breathe after all these years. Decided to start it on the chassis before I lowered the body back on to check for any issues. Was pleasantly surprised to find only one tiny leak at the thermostat housing that formed a drip on the intake manifold. When I installed the housing I used a Felpro gasket and Permatex Water pump and thermostat housing RTV silicone. I’m planning on pulling the housing back off and installing a new gasket. My question to the group is what has worked on your builds? I’m a little concerned that the pressure in the cooling system can force fluid under the RTV which is what happened thus far. I’m wondering about these three options:
1) Try the RTV and Felpro gasket again and hope for a better seal
2) Felpro gasket alone with no RTV
3) Felpro gasket with Type 3 aviation seal which is messy but very tacky
Thoughts?
Thanks,
Jim Tried this option first and had a pinhole leak. Wondering if a better application of this a second will do the trick Wondering if it is best to just use a gasket alone to seal the imperfections in the cast iron manifold and aluminum housing Have had good success with Type 3 Aviation seal on the engine and wonder if this is the best option to maintain a seal with temperature and pressure
Just today I experienced a similar situation after replacing the thermostat on my ‘73 Olds 350. I tightened everything up, added coolant, and, before even starting the engine, it leaked in the area shown by the red circle on both sides. I had used only the gasket with no sealant.
I removed the housing, and the first thing I did was make absolutely sure that the two mating surfaces on the housing and on the engine are as flat as I can get them. Make sure that every last trace of any old gasket and/or sealand is removed.
I then used a new gasket with a sealant (Permatex High-Temperature Gasket Maker), and I put the sealant on both the engine and on the housing with the gasket in between. I then put the housing back in place, making sure to insert the curved tube as far as it will go into the small piece of vertical hose attached to the water pump. I did not tighten the hose clamp here until after I had tightened down the two thermostat housing bolts. This allows the tube to go down into the rubber hose as far as it needs to.
I think this was a mistake I made the first time. I think I tightened the hose clamp on the small tube and then tightened the housing bolts, and that can prevent the side of the housing facing the front of the engine from seating fully.
Anyway, after all the above, I refilled with coolant with no leaking. I started the engine and ran it until fully warm and still no leaks. I’m thinking/hoping I’ve fixed it.
Last edited by jaunty75; July 25th, 2023 at 09:16 AM.
That felpro gasket also fits the 258 AMC, it’s almost the same gasket except the bolt holes are a little elongated. That’s the problem I have had with leaks.
I found a different gasket manufacturer that fit better, but can’t remember who made it or the part number.
Appreciate the assistance! Read through other posts as well and got to work. I did get the Emory cloth out and on a flat surface, removed the pitting on the aluminum thermostat housing as well was cleaning up the cast iron intake. I had used Permatex thermostat housing gasket maker together with a gasket and the failure (weeping) occurred through the gasket material that was between the gasket maker and the metal surfaces. Now that I have everything clean and flat, I going to try gasket maker alone to seal the two surfaces. The right stuff from Permatex looks up to the challenge. Will update after reassembly.
Jim Emory cloth and flat surface Cleaned and smoothed
Those mating surfaces are much cleaner than I was able to get mine. I would think you should have no problems. But I'd still put a gasket between them. GM did recommend it, and thermostat housings do come with them. Just sayin'.
I've seen it written on at least one or several modern day gasket sealant/sealer materials - I can't recall which one (sorry). None-the-less, what I've always done in the past is this. I'll apply the sealer/sealant to the gasket & housing surface (wherever I'm installing a new gasket not necessarily a thermostat housing). I tighten the nut(s)/bolt(s) just beyond hand tight with a wrench ONLY until they're snug - I don't tighten them down and I never torque them straight-away. I generally wait until ~75% of the rated cure time has passed for that particular sealer/sealant. At that point I'll then tighten/torque the bolt(s)/nut(s).
Thanks guys. Norm, the process you follow is one similar to what Permatex recommends for proper curing of the sealant and final torque. I looked back at the PIM to see how the thermostat housing was installed and I’ve included it here. A gasket is used and the bolts are dipped and the torqued to spec (15-20 ft-lbs). No sealant was used. The sealant technology (Permatex as an example) has come a long way and offer sealant, gasket makers, etc. So the real question is what is best used to seal these two surfaces. A gasket alone should work but most of us would feel more comfortable using a sealer along with it (e.g. Aviation sealant). It appears that gasket makers alone are used every day on new automobiles with great success. Think there is more than one way to successfully solve this leaky thermostat housing!
I don't know if anyone has pointed out that you need to use a thread sealant on the bolts.
The coolant will wick up the threads and leak, weather you add sealant to the gasket or not.
Just wanted to provide an update. Applied a thin coat of Permatex Thermostat and water pump gasket maker to both the thermostat housing and intake manifold, finger tightened and waited the recommended hour then torqued to spec (15 ft-lbs). Let cure for 24 hours, added antifreeze and fired up the 455. No leaks.
July 25th, 2023, 08:07 AM
