When the shop installed my Macs on my 97 Z, they put some kind of red sealant on the flanges--I dont know if my headers were warped or what, it wasnt noticed till i got home--I bought the stock metal GM gaskets, i was told off this site those were as good to use on Macs. Thats was a couple of years ago. Dwayne

 

I used 1470 with RTV on driver's side only - didn't on pass. side because I had a lot of trouble with the RTV sticking to everything since I didn't use 2 extra bolts to hold header in place (don't know it that would've helped either way).

 

Skip the Felpro 1470's. They lack the material. Snag some Mr. Gasket Ultra Seals for the LT1 (they have plenty of material... sorry don't have the number).

There should be no reason to use a sealant. I'm using the Mr. Gaskets with Breslin Split-locks on Exotic Muscle 1-3/4" headers in a '96 Vette. Proper fasteners (or a good re-tightening schedule if you choose standard fasteners) is the key. Sealant can actually cause a leak. Exhaust gaskets should be installed "dry".

With a good header flange the stock GM gaskets can work fine (if they cover the header ports).

I believe all this "copper" and "dead soft aluminum" garbage is to make up for crummy header quality and the propensity for the end-user to use sealants.

 

So, I would install them "dry"? Ive read lots of posts where people recommend soaking them in water so they'll swell.

 

Perfect sealing requires two (perfectly) mated surfaces (in our case header flange to head) with the correct clamping force (header bolts).

The two surfaces should retain their shape (flatness) and the retainers should maintain their clamping force.

Since most commercial machining isn't perfect we need gaskets. The less gasket the better.

Realistically, the best surface mating is attained by making sure the surface is very clean (no old gasket material) and is flat as possible.
Cleaning is easy although time consuming, flatness with the head ports is a toss-up (there's not a lot you can easily do), a good, thick, flat header flange is up to the manufacturer.
The last part is clamping force (head bolts). This is up to you. Standard head bolts with a lock washer is pretty good. You may only have to retighten after one or two heat cycles. Breslin Split-locks or Stage 8's normally don't need retightening if installed correctly.

Most guys have problems because of:
1. Bad mating surfaces.... It takes time to clean stuff. Most people don't realize the benefit and don't put in the time.

2. Improper torquing..... Torque to manufacturer spec. "Extra tight" probably means exceeding the yield strength of the bolt which reduces clamping force. Many yahoos out there have no idea what a torque wrench is for.

3. Crummy manufacturer tolerances (crummy header flanges).... Mass marketed headers are hit-or-miss on quality. It's the nature of mass market.
You have to pay attention to the quality of the part you receive.

Regardless of the problem most guys think that "soaking" or "double gasketing" or "crank down on it some more" will work. Usually that's not the case. Proper preparation and good parts are the key.

 

I used Fel-Pro exhaust gaskets, and put a thin coating of orange high temp RTV on each side. That and some $3.99 header bolts from Autozone and I've had no leaks at all.