I have Cliffords 3-1 headers, and the collecter gaskets are leaking again. This time around I used copper ones from summit. Ive tried Mr gasket paper ones, and still no go. Am I the only one thats having this issue? What do you guys use on these headers? Maybe the surface of the collecter is majorly warped.

welcome to the wonderful world of headers. i have the clifford street rod headers and had to stack two paper gaskets per collector to get a good seal for any legnth of time. usually gluing them together with ultra copper helps. a buddy has the dead soft aluminum gaskets on his vette and claims they are gold. a few good whacks from speedbumps should take care of that!

-james

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I just wish collecter ends would have a nice big flange surface like the stock exhaust manifolds. Why the heck don't they?

Ultra copper on exhuast? I used ultra copper on my theromstat housing and it gets soft and sticky (just like it came out of the tube fresh) after the engine warms up. I figure exhaust would make it boil and evaporate.... btw, we are talking about Permatex Ultra Copper, right?

It is a small leak, ordinarily no big deal if I was using a carb, but I think that it might be throwing the o2 sensor off and reaping havoc on the efi setup.

The soft copper gaskets I am using now have a lip on them, I have seen some that are soft aluminum without a lip, I think those will be my next victims.

Get a foot or so (for each collector) of the fibreglas rope that's made for sealing furnaces and woodstove doors etc., and also get a tube of the gasket adhesive made for the rope.
Lay a generous bead of the adhesive around the flange surface of the collector, and then firmly place the rope in the adhesive...(pretty much the way you're supposed to use the stuff on a furnace).
I used this method on a /6 truck engine (stock manifold to exhaust flange) . Put it together at least 3 years ago and still no leaks.

Fiberglass rope eh... never heard of the stuff. I think the main issue is that there is such a small ammount of sealing surface on each end. I'm weary of using any sort of glue because I'm afraid it'd ruin the o2 sensor with its fumes. I could always take the sensor out and let the glue set I suppose.

The glue would be downstream of the O2 sensor, but only you would know for sure.
You may have already said this, but you probably could remove the sensor and allow the glue to cure while the engine runs.

Never heard of fibreglas rope...
I guess if you live in a warm climate you may never have needed to familiarize yourself with things such as furnace repair
You might have trouble even FINDING the stuff I suppose, but you probably could find it online.
Anyway, you really don't need a wide sealing surface, just something rigid and reasonably straight. The rope will seal even on some pretty large imperfections, and because the rope itself never actually hardens, vibration and engine torque will not ruin the seal.
here's an examble of the stuff I'm talking about
http://doityourself.com/store/6859235.htm

After the headers my exhaust pipes come together and join in a Y fashion, then one pipe to the back. My o2 sensor is just after where the Y joins.

Hmm that place has lots of fiberglass rope and silicones, one that even goes to 2000 degrees. Seems promising, silicon the rope to the collecter, put a bit of silicon on the other surface of the rope, then assemble, and mush more silicon all around the collecter and let it bake.

Loghead, curious, which silicon did you use exactly? That site has many brands on it. Also for the rope, when you silicon it on and let it dry without starting the car, will it harden? I was afraid that if the silicon stayed soft, and I torqued down the pipes, the rope would lose its place

circumstances...

The adhesive I used is called "stove gasket cement"
It says it contains alkaline silicates.
It will air-dry, but to cure it, it needs to be exposed to heat...
so you have to fire it up once it's all assembled to actually cure the adhesive.
This stuff cures hard, its the rope that remains pliable.
That's what I used, but there's no reason why you couldn't try something else.