exhaust gasket sealant?

i just replaced a leaking exhaust gasket right in front of my catalytic converters. i took things apart enough to clean the flanges well, and used new hardware. the flanges seemed to be in good shape, and things went together fine, but its still leaking. it really looks like there should be a 4th bolt, but theres not. thats where its leaking, furthest from the hardware. is there any type of paste or sealant that will hold up to the heat that i can use on the gasket? this is what the gasket looks like, just for reference. its leaking around the top left of the pic.

Though I'm not sure what it is, there is some sort of sealant on the cat flange on mine. First time I crawled under the truck I thought it was orange rvt. It is an orange (maybe copper) color, and is squeezed out around the edges.

About the only sealant that will stand up to hot exhaust temperatures (well over 600 deg F for the oxygen sensors to work) is a welding rod. Orange RTV will burn right out., and then make a hole.

If replacing the gasket doesn't help, one side may be warped.

MAKG said:

About the only sealant that will stand up to hot exhaust temperatures (well over 600 deg F for the oxygen sensors to work) is a welding rod. Orange RTV will burn right out., and then make a hole.

If replacing the gasket doesn't help, one side may be warped.

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i know rtv wont take the heat. i wonder if its warped, but its still really solid, not all rotted out. its better with the new gasket, but i can still hear and feel the leak. theres gotta be something i can use....

How about a sanding belt?

copper permatex?

I'm working on my Y-pipe to cat inlet flange too (not able to get the top two bolts off yet - muffler shop says they can't burn them off without damaging the flange), so am interested in this thread.

Thoughts . . .

Possibly taking a flat file to the mating surfaces (if you haven't already) might help? Also, if you can get the cat completely off, the belt sander/machining route?

I'm also wondering if one took some thick copper grounding wire (sheathing stripped off) and made a wind or two of this around the outside of the pipe holes of the flange but inside the bolts, whether this might make a copper crush-fit seal? Perhaps there are appropriately sized copper rings available at a good hardware store?

FYI - I just bought an older car, with an overhead cam engine and deeply recessed spark plugs, that had recently had the valve cover gasket replaced. When I pulled the spark plugs, there was some rubbery black silicone on the electrode, inside the combustion chamber (obviously the guy replaced the plugs while silicone was still oozing out from the gasket, and got goop smeared on the end of the plug). What astounded me was that the silicone was still solidly clinging to the metal of the plug, and was still completely rubbery! I don't know exactly what brand/type this stuff was, but it sure stands up to heat.

One other thought, perhaps use of metal epoxy, but using about half of the proper amount of hardener. This might give a tough, heat resistant fill, but still retain enough flexibility to not crack and blow out?

Kunar, please keep this thread updated with your progress, and if you find a solution.

CraigK
P.S. I will never again use regular bolts and nuts on exhaust systems. For a couple of bucks extra, I use only stainless steel hardware. Bolts are shiny, clean and easy to remove many years later.

I used this stuff called Muffler Mend. It comes in a tube like RVT but it's designed to patch holes in mufflers. I'd never try to patch muffler holes with it but it's great for sealing flange gaskets. You should be able to pick up a tube for a few bucks at your local auto parts store.

CraigK said:

I'm working on my Y-pipe to cat inlet flange too (not able to get the top two bolts off yet - muffler shop says they can't burn them off without damaging the flange), so am interested in this thread.

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If you carefully take a dremel tool with the cutoff wheel attachment, you can cut just the nut off. The flange actually has studs very similar in design to a wheel stud. While it is possible to remove them from the flange, you will probably warp the flange from hammering the sob out, and I dont know if you can get the exact size replacement. As a last resort, just cut nut/stud as a whole and remove the whole pipe assembly and drill out the stud and install a ss bolt and nut in its place. If using the factory studs, you probably want to get new nuts, and do yourself a favor and run a die over them to clean up the threads. I had to do this to one of the bolts on the flange a few weeks ago during my header install. Seals fine like this.

Sevensecondsuv said:

I used this stuff called Muffler Mend. It comes in a tube like RVT but it's designed to patch holes in mufflers. I'd never try to patch muffler holes with it but it's great for sealing flange gaskets. You should be able to pick up a tube for a few bucks at your local auto parts store.

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Yes, It is at your local advance auto parts, $5 in the exhaust pipe/adapter section. Dont ask how I know this. Never used it, but sounds like it might be worth a shot.

Kunar, Are you sure you cleaned all the old gasket material off of the mating surface? A simple wire brush does a good job. Or if you have it off the vehicle and/or have enough room, take a wire wheel on your drill. Makes it look brand new. Can you see any warpage in the flange when its bolted up. Dont be afraid to hit it with a BF Hammer. Worked for me, haha. There are only 3 holes in the flange regardless of year. I guess it really couldnt hurt to put a second gasket in there to layer it up, with maybe alil bit of the muffler mend. Now if that doesnt work...

OK, I don't understand what the problem is getting flange bolts off.

Put it on the rack, wedge a box end wrench on one side, and get a 3 foot cheater (no kill like overkill...) on the other and just break it off. There is absolutely nothing special about those. Go to the hardware store and get three stainless nuts and bolts to replace them.

Oxy heat might warp the flanges, but brute force won't. That bolt will snap FAR before you'll bend anything else.

And the muffler shop can't figure this out? Give them a stick of gum and tell them to walk across the room. See if they can handle that.

MAKG said:

OK, I don't understand what the problem is getting flange bolts off.

Put it on the rack, wedge a box end wrench on one side, and get a 3 foot cheater (no kill like overkill...) on the other and just break it off. There is absolutely nothing special about those. Go to the hardware store and get three stainless nuts and bolts to replace them.

Oxy heat might warp the flanges, but brute force won't. That bolt will snap FAR before you'll bend anything else.

And the muffler shop can't figure this out? Give them a stick of gum and tell them to walk across the room. See if they can handle that.

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On mine the bolts were threaded right into the opposite flange which is pretty thick steel. I twisted each bolt off with the stud stuck in the flange, and was then faced with the task of drilling all the studs out. I ended up just welding the whole thing together.

So the exhaust shop might not want to go through the process of drilling all the studs out and retapping the holes.

I build custom exhaust as part of my job. I have worked in exhaust shops for years. I have never used a drill to remove broken studs, all you do is end up with a hole off to the side. With an acetylene torch (propane doesn't work) you can easily cut the bolts out without damaging the flange. Every 2.9 exhaust flange I've ever seen has been warped. Rust scale builds up between the flanges and warps it badly. I use a small carpenters square (1') and a ball peen hammer. Once I have the flanges flat again I use a generous coating of ultra copper on either side of the gasket. The ultra copper won't blow out if the gap isn't to big.

Thanks all for the tips on removing the flange bolts.

The problem is, the pipes are on the Bronco, and the Y-pipe is NOT coming off (this would be an even bigger job - believe me!). This flange must be disassembled while on the vehicle. There are no nuts or bolt heads left. Just knobs of rust. (Probably original bolts on a 1987 vehicle.) And almost no room to work around the top two bolts. (I was able to cut/grind off the lower bolt on both sides, grind down to the flange, and then knock the bolt stub out with a punch.)

Yellowsplash: I bought special Dremel cut-off wheels and tried this method. It is just too tight to get at the upper bolts. Believe me, I tried.

I then bought one of those cable extensions to a 1/4" drill, and tried using a grinding wheel. (Smaller hand held end.) No luck either. (With the access available, all it did was bounce around.) Now, I've made a cut-off wheel attachment for the drill cable attachment from a worn down wheel from a grinder (~2.5" diameter), and will actually drag my drill press out to the driveway, lay it down under the vehicle, and power the small grinding wheel with this (belts set on highest revs).

I was disappointed at the muffler shop's response. Perhaps I should have tried another shop. I don't see why, with a little care, the bolt heads and nuts couldn't be blown off with a torch without damaging the flange. But if a shop (most shops in my experience) don't want the work, they wouldn't do it properly anyways. And I am not buying a new Y-pipe and cat for a 21 year old vehicle, just because an exhaust gasket is leaking.

CraigK

Mutant Pony said:

I use a small carpenters square (1') and a ball peen hammer.

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i was wondering when someone would post up the easy CORRECT way to get it to seal.

sometimes i use a large cresent wrench on the ears also to bend them back out.a new gasket and not overtorqueing should be all thats needed unless the mating surface is pitted beyond sealing

CraigK said:

Yellowsplash: I bought special Dremel cut-off wheels and tried this method. It is just too tight to get at the upper bolts. Believe me, I tried.

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Hmm...Well my truck is on both a body lift and suspension lift so the body is lifted and the radius arms are dropped some. I guess it would be alittle tougher on a stock vehicle. FWIW, I actually have a cable extention on my dremel tool. The drive end after the cable is only 5-6 inches long. Having this cable allows you to maneuver from the transmission side over top the exhaust and flange to the top bolts. Trust me, its not a comfortable position, but you gotta do what you gotta do. Allow the cutoff wheel to do all the work; dont press down hard cause they tend to break pretty easily. Hope you bought a tube of cutoff wheels. Good luck...