Exhaust leaks, Any tricks to getting the HEs to seat properly? Options

[KaptKaos]

I have had my heat exchangers off and on now about 4 times, trying to make sure that they are seated and tight. I have checked the copper gasket, put in new ones and still I get popping or back firing on decel.

Tonight I pulled the driver's side and flat filed the tops of the HEs to make sure I have a good surface. Nothing drastic, just trying to make it flat and smooth.

Is there any trick to getting everything buttoned up properly?

[rjames]

Do as the Cap'n says. Use one and only one copper gasket for each port.

I also tightened the exhaust nuts slowly in an alternating 'x' pattern to make sure the exchanger was being fitted evenly.
Here is a great link with picutures on the entire process.

Prepping and Installing Heat Exchangers

Also, Gint makes a good point. How do you know that the popping & backfiring is a result of an exhaust leak at the exchanger(s)? I had a huge exhaust leak at the exchanger due to a gasket not being seated properly, and my car never popped or backfired.
Are you sure you don't have any holes in the exhaust system (muffler included)?

[KaptKaos]

Also, Gint makes a good point. How do you know that the popping & backfiring is a result of an exhaust leak at the exchanger(s)? I had a huge exhaust leak at the exchanger due to a gasket not being seated properly, and my car never popped or backfired.
Are you sure you don't have any holes in the exhaust system (muffler included)?

I am pretty sure that there are no leaks in the exhaust otherwise. I bead blasted them and checked them over when I installed them. Obviously, I can't see under the HE tin, but I assume that they are solid.