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Discussion Starter #1
Hello all,

Looking for some advice here...

Car is a 1966 Ford LTD with a '61 390 block and mixed components attached. Last summer I completely ripped apart and cleaned/inspected/painted everything forward of the firewall, including the motor. Flash forward to now, I'm finalizing everything and getting it ready for its maiden voyage, tightening up loose ends, etc.

So, last month, I had coolant on my timing chain cover. I did some investigation, thought it was thermostat housing, replaced that gasket, later find out it was the little hose between the water-pump and intake (I never fully tightened the hose clamp). In hindsight, it was probably the thermostat housing too. This fixed the major dripping on the timing chain cover, however, in replacing the thermostat housing gasket, I created a new leak from the thermostat housing gasket. This was evident from the coolant bubbling up from the mating surfaces/gasket area between the housing and the intake. I assumed I rushed and made a mistake, and I replaced it again, this time, waiting a full 24 hours before loading the system instead of a measly 12. Leaks again. I just replaced the gasket for the third time yesterday, and this morning, after waiting 24 hours, I tested it again to find out it still leaks!!!

I've been using felpro gasket so far, so maybe it's time to switch to something else? Quite frankly, I'm tired of draining this coolant system on a weekly basis...

My process:
  • drain the coolant system
  • once drained, pop the thermostat housing off, dumping another gallon of coolant on the floor
  • peel old gasket, and scrape off remaining silicone
  • lightly sand both surfaces to scuff in preparation of new gasket/silicone
  • wipe down mating surfaces first with alcohol, then with brake clean
  • apply light coat of silicone to thermostat housing, place gasket, then lightly coat gasket
  • bolt on thermostat housing, torque to 20 ft-lbs
  • wait 24 hours to cure
  • fill system with fluid
  • test system to find out it still leaks
  • wonder what I did to deserve this punishment

As a side note, today when I ran the car and it leaked again, the upper radiator hose was pretty hard. There was quite a bit of pressure, comparable to grabbing a full bottle of Poland Spring. I've never grabbed the hose when hot before, so maybe this was normal, but I wasn't sure if it was standard to have that much pressure in the cooling system.

Any advice is helpful, even if it's just a new brand of gasket.

Much appreciated as always
-'66
 

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I would advise using Permatex "The Right Stuff" for that (and most other) gaskets for a reliable seal. That stuff actually expands into tiny features that may be trouble spots for normal gaskets and sealers. It's possible that you have a small crack or pit or a warp of the housing from overtorquing in the past. Check for trueness with known straight edge and either replace or true up the gasket surface before reinstallation. Another issue with thermostat housings is the thermostat itself. They can fall out of place slightly and the thin lip of the flange can get caught between the housing and the block/intake, thus preventing full seating of the surfaces. It's not a bad idea to use some sealer on the thermostat itself and lightly "glue" it in to the housing to make sure it stays put on assembly.
 

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I would advise using Permatex "The Right Stuff" for that (and most other) gaskets for a reliable seal. That stuff actually expands into tiny features that may be trouble spots for normal gaskets and sealers. It's possible that you have a small crack or pit or a warp of the housing from overtorquing in the past. Check for trueness with known straight edge and either replace or true up the gasket surface before reinstallation. Another issue with thermostat housings is the thermostat itself. They can fall out of place slightly and the thin lip of the flange can get caught between the housing and the block/intake, thus preventing full seating of the surfaces. It's not a bad idea to use some sealer on the thermostat itself and lightly "glue" it in to the housing to make sure it stays put on assembly.
All agreed, and in my case a flavor of Permatex Gray works for me. Nearly forty years ago I cracked a thermostat housing on a friend's 302, so I can assure that such an event is a possibility, though I bet something above is more likely...
 

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Never had a problem with Stant gaskets, but can't argue the instinct to try other brands.

To the point above about pressure, many auto parts stores have pressure testers that can be borrowed to test the system as well as the cap. Speaking of caps, I used a 16 pound cap on my '66 289 Mustang on track days without leaks.

I AM surprised to lose so much coolant when the housing is pulled after draining the radiator. Ouch. Maybe I've been lucky all these years.
 

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Sorry - one more; fwiw My '62 and '64 Ford Factory Service Manuals specify 12 - 15 pounds of torque for all V8 Engines. same 12-15 in the '66 Comet/Fairlane/Falcon/Mustang FSM for all engines.
 

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Make sure and tighten it evenly. Don't tighten down one side and then the other. The gasket alone will not seal. Top side of radiator will see between, 15-30 PSI, depending on your system setup. That's why that hose, is the one that blows out 95% of the time.
 

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Just to add another tip: I dont use just a bead of sealer, but instead I like to entiely coat the surface of the gasket. I start with a small bead and then with latex gloves on, I smear it all over the surface to fully coat it until no dry spots remain. Then I also use that same sealer on the threads of the bolts , since they can possibly also be a source of leakage.

And, it is normal to feel pressure in the upper radiator hose when warm. I'd be more concerned if it wasn't firm, to be honest. I would say the feel is like an inflated football, maybe slightly less. You may have excess pressure, but I'd think something else would blow before the thermostat housing.
 

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Discussion Starter #9
Thank you all for the replies! I've since ordered a new radiator cap with a 13 lb pressure rating. My current cap is a no-name brand parts store special that was on the car when I got it - figured it can't hurt. Also, for reference, I've been using Permatex red rtv silicone for my gaskets using the method recommended by Jazzmeister, where I spread the silicone over the whole gasket including the bolts. Also, I do keep my thermostat 'glued' in place with the red rtv to prevent it from falling out and/or messing up the sealing surface.

So, going forward (i.e. next week when I take this apart again), I've ordered a tube of The Right Stuff that I will use on the next gasket. As for the gasket, pkevins messaged the Stant gasket. Before I pull the trigger on that, any other recommendations?

Thank you all for the input!

-'66
 

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The right stuff is a bitch to scrape off. Before I use that I would try using four gaskets.
 

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Make sure you tighten evenly and that the bolts aren't bottoming out. Look for not only the overall depth of the hole, but also the shoulder of the bolt if not original Ford bolts.

Also look close for cracks around the intake bolt holes and the housing itself. It is very easy to crack the housing, might have been cracked the whole time

Never a bad idea to sand the housing on a flat block too, to see if something isn't flat.

I would NOT stack gaskets, good way to break a housing. If your housing is crooked, buy a new one
 
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Not trying to be pedantic, but do you have the thermostat installed in the correct “direction?” And properly seated in the groove before you out the gasket in place?

- John
 

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I just sand and clean both surfaces good, use spray copper sealer on both sides of gasket. Place thermo in neck, stick gasket to neck and install.
You can use any and all of the above, but nothing better than 2 flat clean mounting surfaces.
 

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Discussion Starter #14
I ordered 2 new gaskets, and I found that Permatex makes a specialty coolant system gasket material for waterpumps and related components. I'm sure it's just the same stuff rebranded, but what the heck I'll take any advantage I can give myself.

So my plan for this week is to rip that thermostat housing off and try a new gasket and the permatex waterpump silicone.

In the event that that final solution fails, I'll resort to 'The Right Stuff'.

I will also be checking for warpage and cracks on both mating surfaces and double checking the thermostat.

Keep me on your thoughts as the week progresses haha - fourth times the charm.
 

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Discussion Starter #15
Update: I drained the cooling system this morning and pulled the water neck. I cleaned off the old silicone, hit it with a wire wheel, and checked for flat. I think I found the culprit... the mating surface has cupped enough where I think it's not clamping properly in the center. Its not much (eye balling it, maybe 1/32" - enough where I can slip my license underneath) but it may be enough to cause an issue.

I called my local parts stores, and all anyone has is the chrome filler necks that can rotate to any angle and seal with an o-ring. After reading several scathing reviews, I decided I'd rather be patient and pay for shipping then deal with this cooling system leaking again in 3 months time. I've ordered a four seasons cast thermostat housing (i needed one with the port for a temperature probe for my under dash gauge), and I should see that by the end of the week.

Wish me luck!

-'66
 

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There was never any sealant involved on factory installation. A cast ridge in the housing provided an almost like O ring seal with gasket when mating surfaces were smooth. An original Ford thermostat had locking tabs to hold in housing. Surprising how low of torque is called for on small bolts and how high for larger ones. I know post is almost 2 years old, keep the silicone in the drawer and follow the shop manual specs. Don't crank every bolt till it won't turn anymore.
 

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I called my local parts stores, and all anyone has is the chrome filler necks that can rotate to any angle and seal with an o-ring. After reading several scathing reviews, I decided I'd rather be patient and pay for shipping then deal with this cooling system leaking again in 3 months time
-'66
Well crap. I didn't read any reviews and just bought the Dennis Carpenter aluminum one. This is for a small block so hopefully I have good luck with it. I had one made by Aeroflow (made in Australia I think) prior to this one that had the same issue you described. Always seems to leak when it cooled down.

00D1ACC4-0900-4211-A798-68BA50141159.jpeg
 

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Well crap. I didn't read any reviews and just bought the Dennis Carpenter aluminum one. This is for a small block so hopefully I have good luck with it. I had one made by Aeroflow (made in Australia I think) prior to this one that had the same issue you described. Always seems to leak when it cooled down.

View attachment 168514
Your Billit aluminum one should be good.

It's the cheap cast and chrome plated one's that are garbage and leak.

I have trouble believing they still sell those chrome junk ones.

I also despise silicone.
The factory didn't use silicone and they didn't leak.
If your parts are good and no cracks there is no need for silicone.

Some guys just don't get that, and slather the stuff on and make an ungodly mess.

I work at rebuilding transmissions, and I have worked with guys that put silicone on EVERYTHING!
It just makes me think they have very poor workmanship skills to assemble something, and be so insecure about the work they do for fear of leaks.

If you do the job properly... and the key is "knowing how to do the job properly" you should never need a crutch like silicone.

Yes, yes... there are times when the part is boogered and pitted and it's just some insurance.
But really... the "proper" thing to do is get parts that are not boogered up, and again... do the job properly.

Ok rant over...

... oh, and yes... there ARE places where silicone is actually needed.
eg, the intake corners.... things like that.
But still no need to make an unholy mess of it....

Ok, now the rant is over... ;)
 

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Whew! Glad I didn't use any silicone......
 

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Ford had there own specification for silicone back when. Yes just a dab here and there. Silicone and gaskets do not mix. Original post said 61 block in a 66, how is the block attached at motor mounts and the alternator mounting? Just curious, Ford allowed for backfitting but not forward. Pre April 67 had large Tstat, post April had small. The change affects both intake and Tstat housing.
 
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