Leaky exhaust manifold gaskets.

[1969chargerrtse]

I keep blowing the gasket from the cast Iron manifold to the block.  I used the special type that are 3 soft pieces of aluminum and that to also lasted just one year.  I realize the cast exhaust manifolds are 40 years old, is it possible they warp and I need to pull them and have them machined/planned  down flat and smooth.  Anyone run into this issue and have theirs machined?

[BrianShaughnessy]

Put a straight edge to it.

Had a friend's GTO manifold's shaved awhile back. 

[1969chargerrtse]

Put a straight edge to it.

Had a friend's GTO manifold's shaved awhile back. 

Point being it is common?

[no318]

I always have the machine shop surface the manifold and put it on without a gasket.  Has worked well for me.  Make sure that an experienced machinist does it.  It is possible to machine them on an angle and really mess things up.  Hate to ruin some HP manifolds.

[Sublime/Sixpack]

Did you have exhaust leakage there before you put the gaskets on? Is that why you're using gaskets?
I ask because the factory didn't use gaskets there. I've never used gaskets between the exhaust manifolds and the engine on Mopar big blocks, and never have had leaks there.

[Nacho-RT74]

I simply use a coat of High temp silicone... I know will burn anyway but still burnt will seal some posible porosity on matched surfaces

[CYACOP]

I had the same problem. Mine looked straight with a straight edge, bolted them up without a gasket and they sealed fine.

[375instroke]

If you have a big file, you can run it on the sealing surface and see what the file touches.  Run it the length of the manifold, so it's touching at least two ports at a time.  It's easiest to see with the manifold still dirty.