Intake Manifolds...Sealants or not?

[Kevin68N71]

Beyond frustrating.

I feel like I have woken up in some alternative universe.  I have had my 68 since 78, engine out twice, heads off a few more times, and every time I have reinstalled the intake manifold I have thrown on a nice, dry, clean, new valley pan gasket, tightened down the hold downs front and rear, lowered the cleaned up manifold, torqued bolts in order and to spec.  I never used any sealants or any other stuff.  Now, everyone seems to do this.

The issue started a few weeks ago, when I noticed the car was a bit down on power.  Smooth and everything, but lacking a bit of punch.  A few weeks later, it would not start.  I swapped out my year-old Edelbrock AVS2 with an older Carter I had, still no start.  I was shocked to find that three of the passenger side intake manifold bolts were loose.  Not completely loose, but I could almost take them off with my fingers.  Tightening them down, I got the car running on the old Carter, but huge vacuum leak and running horribly.  I traced it to the passenger side intake, mostly the front and that center "port". One spray of carb cleaner and my RPM went up a few hundred. 

So, I pulled the manifold.  I didn't see any obvious problems, so I ordered a new valley gasket, and put on the little tube of sealant that came with it.  Got all brand new cat 5 bolts.  Same issue, same leakage.  Took that all off, replaced the gasket AGAIN with that Permatex Right Stuff.  Probably the wrong one, as I didn't notice until I was done that it wasnt to be used around gas!  This was the black stuff.  I put it on thoroughly on both sides of the pan gasket.  SAME PROBLEM!  I don't chalk it up to even maybe being the wrong sealer, as one would think even if it was wrong it would at least seal for a few minutes.

The drivers side seals just fine.

Do these cast iron manifolds ever warp?  If they did, would not the other side leak?  I am tempted to try the gasket/pan/gasket sandwich style, but the thought of gooping up four sealing sides with that junk and attempting to line everything up sounds horrible.

When the manifold was off I cleaned everything, and their didn't appear to be any cracks or damages.

This seems really odd...brings back memories.  My son dragged home an old BMW 2002 that had an awful idle.  We pulled off the manifold and it was loaded with blue sealer that also did not seal.  We never got that thing to run right, and it had so man problems we just got rid of it.  But I have never had any manifold issues with all the big blocks I have worked on.

Any ideas or thoughts?   This is a 1971 383 engine, bone stock, 4bl carb.

[Kern Dog]

If you trust Rick Ehrenberg of Mopar Action magazine, you'd use some type of non hardening sealant on the threads of the intake manifold bolts. HE call them capscrews though, which annoys me even if it is technically correct. I see a threaded fastener with a hex on the end and I think bolt, not screw.
The intake manifold bolts thread into the heads but are open to the lifter valley. Without sealant on them, oil vapor can get pushed up and by that logic, so could vacuum.
Intake manifolds can warp but I'd think an iron one would be less likely to do so compared to aluminum.
The valley pans are often installed without paper gaskets around the ports. I use them though. Some put a bead of RTV around the ports instead. Some install the valley pan dry with sealant only at the front and back of the valley pan where it lays on the "wall".



What are the chances that your intake developed a hairline crack on the side it leaks?

[Kevin68N71]

I appreciate it Kern.

I have tested with carb cleaner everywhere.  The only place I can detect vacuum is at the front two (passenger side) ports, the center port, and a tiny bit, if any, at the rear of that side.  I blasted everywhere else.

The driver's side has absolutely no leaks and it was prepared the same.

If I had a crack, which I didn't see anything obvious, I would think it would just be at that location, not all locations on the passenger side.  The heads/ports and the intake ports looked surprisingly nice, especially after cleaning.

What are your thoughts of using the kit that has the four paper gaskets AND the valley gasket and putting that whole thing together dry?

[Kern Dog]

I appreciate it Kern.


If I had a crack, which I didn't see anything obvious, I would think it would just be at that location, not all locations on the passenger side.  The heads/ports and the intake ports looked surprisingly nice, especially after cleaning.

What are your thoughts of using the kit that has the four paper gaskets AND the valley gasket and putting that whole thing together dry?

I have aluminum heads on one big block and iron on the others. All have aluminum intakes. I use the paper gaskets on both sides of the valley pan intake port areas. I use a high tack adhesive to hold them in place but once they are in and the intake is on, nothing will move then anyway.
Years back I had detonation issues and was chasing my tail looking for the cause. One effort focused on oil seepage/migration up from the lifter valley-through the intake bolts and possibly into the ports. I tried everything that I could think of.
I'm not trying to worry you with some of these comments, just trying to offer some ideas.
Do you have a spare intake to try?

[Kevin68N71]

No your comments are not worrisome. I know I have to take it off anyways.  What adhesive did you use to hold the "sandwich" in place, and did you also use sealer to did you just put it on dry?

I am not understanding why the damn sealer is not sealing!

[Kern Dog]

I used Gasgacinch.

[b5blue]

permatex aviation #3   

[John_Kunkel]

permatex aviation #3   

AAAAGH!! the "Brown Death".