Is there any type of gasket used when attaching the exhaust pipe? I don't see one available in parts books.
I tried some of the rope they use for sealing around the glass on the front of gas fireplaces. It lasted a few minutes before it turned to powder. The best thing is to tighten the nut with the proper "exhaust nut" wrench really good and tight and then if you're still concerned about it put a hose clamp under the nut so it can't back off. When properly tightened the exhaust pipe has very little chance of leaking.
I took some tin foil of all things and stuffed it in just to keep down the leaks . rolled it up and stuffed behind the pipe flange with a small screw driver. I have no leaks from the nut,but I do have tight. Dusted manifold threads with dry graphite for locks snugged tight, ran engine for five mins and tightened up some more. Once the packnut expanded a little from heat it snugged a little more.
There are two types out there hex brass and round steel. The round steel type requires a pipe wrench to tighten. With either style they shouldn't be over tightened particularly the hex brass as it can crack, i've had two cracked brass nuts from over tightening. If the pipe is aligned with the manifold and clean, moderate tight should be sufficient. I believe I have seen a third style with "ears" that tighten up as a clasp as opposed to thread torque.
There is no gasket. All you need is a packnut with good threads, and a pack nut wrench. Be sure to start the nut onto the threads by hand. Go as far by hand as you can. A little lubricant on the manifold helps. After that tighten the pack nut with the wrench while wiggling the exhaust pipe with your free hand. You will feel the exhaust pipe seating on the manifold as you do this. Get it as tight as you can. I have never had one come loose on any of my cars in over 50 years of driving.
Have a look at the flair on the exhaust pipe too. Mine was thin due to heat and age and the nut would often come loose. The weak metal eventually cracked. I bought a new pipe from one of the vendors. The new, thicker metal tends to function like a washer and it does not come loose now. BTW, despite what you may have heard about those new pipes, mine was a perfect fit in every way.
Larry, I am glad you never had a pack nut come loose in 50 years. I have had one come loose on one or more of my cars almost every year. The clamp that the venders sell does solve this problem. It clamps to the exhaust pipe and has a tab that rest against the nut that keeps it from turning. Sometime with a hose clamp, the nut will still turn loose.
The clamp will deep it from turning if the threads are not stripped. If the threads are bad it will just pull straight out.
Norm
Willie - I make my own "clamp" with a little piece of strap iron and a hose clamp. About an inch and a half length of very light strap iron with a bit of a kink in it for an offset just high enough to rest on one of the "flats" of the brass exhaust nut and it works great. And I think mine look better than the ones the vendors sell,.....not that the appearance matters much,....FWIW,.....harold
Willie - I make my own "clamp" with a little piece of strap iron and a hose clamp. About an inch and a half length of very light strap iron with a bit of a kink in it for an offset just high enough to rest on one of the "flats" of the brass exhaust nut and it works great. And I think mine look better than the ones the vendors sell,.....not that the appearance matters much,....FWIW,.....harold
Willie: If you tighten the pack nut as I just described, it WILL NOT come off. If it does, you have a inferior pack nut, or the threads on your manifold are no good. I have seen inferior reproduction pack nuts in the past, and it could be you have one. The ones Snyders and Langs sell are probably better than original, you know me, I like original stuff.