I've been having traumas with my exhaust and getting it to run leak free.. I ended up designing the pipes so they came off the manifolds of my 305, and then droped under the frame, and then backwards under the rockers into flowmasters.

It took me 2 days to figure out why they wouldn't originally seal up, and it was because I had one manifold that took a flat gasket, and another one that took a doughnut shaped one.

Driving to school 2 days ago, the doughnut shaped one blew out.. or in or something, and is now not sealing the horn on the manifold.

Am I doing this right.. using the correct gaskets?

I'm sure I've got the right size.. does anyone know the ins and outs of these things? I thnk I've tried almost every type of gasket and doughnut the parts store sells.. and at about $12-$15 a shot, they're expensive to replace.

Here's a picture.. i guess its like a cut away or something..

I think what keeps happening is that the exhaust heats up, and the gasket goes soft.. and then either shifts or collapses into the exhaust tube..

Can I just stick a standard thin gasket in there instead of a doughnut?

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This is the kind of gasket im using..



Its a wire strip thats wound around and around and then covered in some soft metalic stuff thats kind of mushy.

more pictures to describe this more.



This is the passanger side manifold.. (not mine.. but mines the same.)

this is from a 1985 z28.

the part where the downpipe goes into is a flat surface thats chamfered inside... presumably to accept the gasket.



This is a drivers side manifold.

See how the portion where the exhaust downtube attaches is beveled out.. so that the opened end of the horn will seal right up.

All I used on that side was a simple foil gasket thing.. and it works perfect.

anyone have any ideas what kind of gasket/special precautions I need to take to get the passangers sdie to seal up?

I once had a 1989 chevy truck, with cast iron manifolds. It had doughnuts on both sides, and they both leaked...(they were old and I didn't care)

have you tried a doughnut gasket that has a fire ring inside of it. That is a metal sleeve that usually protudes out of the flat side.

No I haven't. I sized them up, but I never bought them because I figured the inside of the manifold was slightly dished.. so a beveled gasket would work better then a flat one.

BUT.. I now remember that my setup originally had a butterfly inside a chunk of cast pipe that bolted to that flange.

Which might make it very hard to seal without that portion on there because I doubt anyone designs something like that.

On another note. I dropped the downpipe this afternoon in a parking lot :P and found that the gasket had unraveled itself.. all of the mushy metalic stuff had been burned off, and all that was left was a strip of wiremesh..

I'm going to try the gasket that Mav mentioned..

By chance, does anyone know what kind of nut (thread size/type) the stock manifolds use? I've been using a metric nut that seemed to fit, but then when cranking it up today I stripped 3 of them in a row.. it seems that they dont fit well enough.

Anyone know what the thread type is?

The pipe that uses the doughnut gasket sometimes has a more complicated end on it - a straight piece of tube extends a half inch or so above the flare. The gasket slips over that stub pipe, which then protects the gasket from direct contact with the exhaust gasses.

But then other pipes don't have that feature. Maybe the gaskets are made differently depending on the configuration of the pipe. I think that the gasket I last used on my 258 looked like yours but had a thin sheet metal collar that covered the inside of the gasket and wrapped a little way around the top and bottom.

Looks to me like the right side is made for a FLAT gasket, even though the inside is chamferred a little. The headpipe flange must also be flat. To get flat flanges to seal, you have to loosen the rest of the pipe so the flat flanges can line up without trying to bend or twist before tightening.

But - the part you removed - the heat riser - the cast thing you tossed away - it's designed with both ends different so you can't toss it without changing the headpipe flange.
Get another one with the proper gaskets for it and it'll work fine. They usually are flat flanges on top at the manifold, and flared on the bottom.
No matter what type of gasket you use it's not going to seal when you mate different types.

Nuts - best to use brass nuts, available at any muffler shop. They don't rust on.

[ QUOTE ]
best to use brass nuts, available at any muffler shop. They don't rust on.

[/ QUOTE ]

. . . as quickly as steel nuts, but they're no guarantee of easy removal. The steel stud can rust and the brass nut gall to it. Use plenty of anti-sieze. It's no guarantee either, but it will also help.

True, but at least they are "less worse."

But unfortunately, not as good as "more better."