390 exhuast manifold help

ok so I was able to get the exhuast manifold off today with out breaking any bolts! I just did the p***enger side, but have been soaking the bolts on the other side in prep for replacing that gasket as well. so heres the question if I get the exhuast manifold machined to be sure it is flat, I understand that I dont need a gasket.the muffler shop said to just use a high heat silicone on it. whats everyones take on this?

 

I would try it without a gasket or sealer...see if it sounds ok no puffing or ticking sounds.

 

I use high heat aerospace RTV silicone sealer. At the locations were the upper row of bolts broke, just drill thru and put in a bolt/nut.

 

I would use a gasket due to one imperfection will eventually get bigger and bigger.

 

What does the head surface look like? Ford used gaskets on these engines and they lasted for many years before they needed replacing. That's why they were such a ***** to fix. I'd get a quality set of exhaust gaskets (mainly for the look) and if the manifold is flat they should last 20 years like the factory ones did.

If you never want to open the hood, silicone will probably work but it looks like ****. IMHO

 

Ive been told that you are not supposed to use a gasket in this. Why would that be? I personally feel that a gasket will help seal bettween the 2 surfaces. I took the exhuast manifold down to be surfaced today. the head surface looks good and I understand that they dont warp like the exhuast manifold.

 

The head doesn't warp in that area but if it was leaking for a long time the escaping gases can actually wear away the cast iron surface so that it won't seal. The sealing surface of the head is pretty narrow compared to other engines.



The end ports are the ones that scare me. The sealing area is only about a 1/4 of an inch wide or so. Even the corners of the center ports don't have much sealing area. Anything that comes out of a tube won't hold for long if everything is not perfect in those areas. Ford designed them to use a gasket and I'd personally stay with a gasket. You might see if they make a copper gasket for the FE. It's more expensive but so is changing gaskets.

Chevrolet is a different animal. No gaskets required.

 

now you got me curious, so I looked in the only Ford shop manual I have that talks about an FE engine, the one for the 1962 Tbird. What do you know...they did NOT use gaskets between the head and manifold! instead, they say to coat the mating surface with graphite grease, and make sure to bend the lock tabs over the bolt heads after torquing them to 12-18 ft lbs.

Gaskets insulate the manifolds from the heads, so heat cannot transfer into the heads. I am pretty sure that a sizeable number of warped and cracked manifolds are cause by people putting gaskets on....thus helping the manifolds overheat.

Just another one of my hare-brained ideas.....

 

so still whats better? gasket or no gasket?

 

no gasket is better, as long as the surfaces are flat enough to seal. If the head or manifold are warped or eaten away, and you can't get a good seal with no gasket or with just high temp RTV, then you'll probably need to put a gasket on.

 

ok so some black high temp RTV it is. I had the exhuast manifold surfaced so that part should be flat. Now do I have to give it full cure time before I check for leaks?

 

probably...

 

Use the high temp rtv sparingly and let cure for a full 24 hours cure. This stuff works great IF you clean both surfaces well and tighten properly and let fully cure before fireing up engine. I have used it many times without fail!

 

I had the same deal on my 64 T-bird. High temp RTV works great, but I had to re-do the install with a gasket coated in RTV on both sides due to worn spots. Manifold was flat, and the head looked the part, but it was not. The fully cured RTV heald for about a block. With the double coated gasket, it held for about 40000 miles until I sold it. Should still be holding. Try the RTV first, though. You'll find out soon enough if it isn't enough.

 

get the straightedge out and check the mating surfaces on the heads to be sure. when they leak for extended amounts of time they can become warped. in the 80's i worked for a ford remanufacturing company and it was standard procedure to surface all 390 ford exhaust flange surfaces. graphite on gaskets helps seat gaskets and don't overtighten the manifolds or your going to be doing it all over again, very common mistake. good luck.

 

check manifolds with a straight edge,if its perfectly flat,run copper gaskets,i have had many older fe's with that annoying exhaust leak