View Full Version : Exhaust manifold leaks

09-30-2015, 08:05 PM
I am having a difficult time trying to get both manifolds to stay leak free. Short of buying used that may also be problem some or buying new from a restoration site that would cost a lot of bucks. What have others done to resolve the leaking issues.
My mechanic has planned them as smooth as he can and has also double gasket them. The double gasket seems to help for a short while but not a log time at all. He tells me that they are suppose to have a raised boss to them at each port. I did not know they had a raised boss to them but... His only suggestion is to get a new set.

09-30-2015, 08:25 PM
They're supposed to be metal to metal without gaskets. If planing didn't work then they may be too far gone. You can try RTV high temp sealant but that probably won't last any longer than the gaskets.


09-30-2015, 08:37 PM
The Squarebird did not have gaskets originally on the 352 (same block as the 390). I'm pretty sure the only change to the Bullet Bird was where the exhaust attached - they went from a flat gasket on the 352 manifolds to a doughnut style on the 390. My guess is the gaskets are the problem - at least it was for me.

After surfacing the used manifolds I bought and installing them without gaskets - it has been fine with no leaks for about 5-8K miles now. The first time I installed them I used gaskets and they lasted about 1K miles before leaking.

Check this old post. http://www.squarebirds.org/vbulletin/showthread.php?t=16727 (http://www.squarebirds.org/vbulletin/showthread.php?t=16727&highlight=manifolds&page=2)

Hope that helps.


edit - ah ha. I see John has the answer on the gaskets before I got my response posted....

09-30-2015, 11:57 PM
I concur. NO GASKETS
The mating surfaces must be flat. The only way to do that is by taking both manifolds to a good engine machine shop.

You live in a good area with plenty of machine shops. Call around for pricing. The actual machine time is only a few minutes but the setup time costs more. Cast iron is VERY easy to machine.

Gaskets don't work because they cannot take the shock. When your exhaust valves open, it's like a shotgun blast of heat and pressure. Cast iron can handle it until it warps. Manifolds usually don't warp unless bolts around the ports fail.

If you have maintained coolant in our engine, the heads should be straight. If it ever overheated, that's when things warp. - Dave

Joe Johnston
10-01-2015, 08:33 AM
No gaskets here too!The mating surfaces were checked and machined true. The metal heat shields were coated with antisieze and the manifolds properly torqued evenly and slowly in several steps. I re-torqued a couple of times after 3 or 4 heat cycles before securing the nuts with the stock sheet metal keepers. I found these keepers necessary as the manifold's nuts were loose when the engine was torn down for rebuilding (sort of a good thing at tear-down!)

10-01-2015, 08:44 AM
Flat mating surfaces as others have said, and just a thin layer of RTV copper upon assembly.

10-15-2015, 08:33 PM
After removing the driver side manifold, I found unevenness in the surface (warping) along with a big chunk out of one of the outlets. In other words the port was missing a corner of the matting surface. I then decided to try a set of Headers. I am waiting on those to arrive.

10-30-2015, 05:37 PM
Are you going to get the FPA headers? I just had a set arrive the other day and they look fantastic in fit, finish, and ease of use. They even put notched stud hangers on them so they can "technically" be installed on the car, rather than pulling the motor, but that doesn't look like much fun to do.