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Sealing no-blow exhaust gaskets

00Dan

Well Known Member
When I installed my exhaust on my new engine I used new Rapco 77611 gaskets on each flange. After an hour of operation I noted some white blow by on the bottom of three cylinders directed towards the spark plug. Is this just a torque issue or can I reinstall these gaskets wet with some high temp RTV? I torqued the exhaust nuts I could get the wrench on to 180 in-lbs, for reference.
 
When I installed my exhaust on my new engine I used new Rapco 77611 gaskets on each flange. After an hour of operation I noted some white blow by on the bottom of three cylinders directed towards the spark plug. Is this just a torque issue or can I reinstall these gaskets wet with some high temp RTV? I torqued the exhaust nuts I could get the wrench on to 180 in-lbs, for reference.

All nuts need to be torqued with pipes in a neutral position. With a three out of four failure rate, I would say they were installed improperly.
 
And High-Temp RTV just isn’t “THAT” high-Temp - you’ll just make a mess, and not seal anything…..
 
As Ironflight says - RTV is not a good choice in this location.

You shouldn't really need any additional products in that area with new cyclnders and a new exhaust because the mating faces should be nicely machined and square. Ensure that the installation of the exhaust has been done correctly - that's to say that the entire assembly has been fitted up and loosely bolted on before tightening any of the nuts. Always tighten the inner nuts first and do the outter nuts last. The reason for this is the flange can float a little and if your socket is a little heavy in the wall thickness then at least you can get the back nut tight and use a spanner on the front nut. If you tighten the front nut first you might find you can't get your socket onto the back nut.

If you have to use anything, a light smear of Maniseal is the best product to use here and I would only use it on the face between the cyclinder and the exhaust flange. The No-Blo Gaskets are supposed to crush up a small amount as you tighten the nuts so if eveything is in good order you shouldn't need much, if any Maniseal in there.
 
Exhaust

The exhausts with a slip join a few inches below the cylinder probably seal best.
Hand tighten all the nuts. Then 75% of torque, then full torque.
 
And High-Temp RTV just isn’t “THAT” high-Temp - you’ll just make a mess, and not seal anything…..

Disagree.

Blo-proof gaskets are not "blo-proof". I will not put an exhaust gasket on without ultra copper RTV smeared on both sides. Holds up fine, seals as intended. The machined surface at the exhaust port is typically coarse. The RTV takes up the gaps and prevents erosion from starting. On exhaust ports with existing erosion, which is very common, it becomes even more important to do this.
 
I'm with rocketbob

There is a guy over on the Beechcraft forum named Lew Gage and he is a proven and recognized expert with 50 years of experience and all the right credentials. He recommends the Ultra Copper RTV so that's what I use too.

I would also agree with jrs14855 in that stepping the torque up until you get to the final torque is critical as well.
 
Disagree.

Blo-proof gaskets are not "blo-proof". I will not put an exhaust gasket on without ultra copper RTV smeared on both sides. Holds up fine, seals as intended. The machined surface at the exhaust port is typically coarse. The RTV takes up the gaps and prevents erosion from starting. On exhaust ports with existing erosion, which is very common, it becomes even more important to do this.

That’s fine if you disagree. I’ll only point out two things:

1) Permatex Ultra-Copper RTV is only good to 700 degrees F (most EGT’s measured 2” away from the exhaust flange run about 1400 degrees).

2) Exhaust manufacturers like Vetterman make no mention of sealant with the Blow-Proof gaskets.

Got a reference other than “that’s what I do”? I am honestly happy to learn more if there is an authoritative reference source.
 
I’ve never had an issue with the no blows. They are extremely soft and thick as we all know. I have also never heard of using any kind of sealant nor have I ever needed to.
I suspect the exhaust nuts were not torqued properly. As was mentioned earlier. Start at a lower torque and alternate from one side to the other to insure the flange stays square. If that doesn’t do it, check the exhaust flange for square and against the cylinder with no gaskets and make sure they fit against each other with no or minimal gap.
I’m sure there’s a spec somewhere for how much gap is acceptable.
L
 
1) Permatex Ultra-Copper RTV is only good to 700 degrees F (most EGT’s measured 2” away from the exhaust flange run about 1400 degrees).

Why doesn't the inside of the exhaust port melt since 1400 degrees is well above the melting point of aluminum?

Ultra copper works, because it does. The manufacturer tells you to use silk thread and run rich of peak.
 
1) Permatex Ultra-Copper RTV is only good to 700 degrees F (most EGT’s measured 2” away from the exhaust flange run about 1400 degrees).

Paul, with all due respect and this is a serious question...wouldn't the temperature that the RTV is exposed to be closer to cylinder head temperature than the temperature of the exhaust gases going past that sealed and clamped joint?
 
Just an example.

One might be surprised at the durability of RTV near combustion temps. My employer had 110 diesel engine test cells. One was mine. Cylinder pressure transducers were located in the head (4 valve) directly exposed in the combustion chamber. The transducer life was extended by several hundred hours by a small recess and application of RTV up to the head plane. I was quite surprised to learn this little fact. This would indicate the surface temperatures are quite a bit lower than combustion or exhaust temperatures.

Edit: Paul has a good point below - why is it leaking in the first place? The exhaust flanges and head flange should be checked for flatness. If there is no metal/metal contact, then the pipe flange will quickly heat and attempt to approach exhaust temps. The aluminum head will always be a lot cooler -and it has the fins. If the non-contact areas are small, then the RTV could remain within its operating temperature.
 
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Paul, with all due respect and this is a serious question...wouldn't the temperature that the RTV is exposed to be closer to cylinder head temperature than the temperature of the exhaust gases going past that sealed and clamped joint?

That’s a good point - anything squeezed off inside would burn away pretty fast. And if it isn’t exposed to the hot exhaust gas directly, then….there isn’t a leak in the first place?

And I’d still like to see some documentation from the folks who build the engine, exhaust, or gaskets that recommend a sealant.
 
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And I’d still like to see some documentation from the folks who build the engine, exhaust, or gaskets that recommend a sealant.

Its accepted practice to use a sealant on NPT plugs such a oil drain plug.
Nowhere does any Lycoming document specify what sealant is appropriate for a drain plug. What does one use then? Or not?
 
Try Fiberfrax

I used the 1/8 in Fiberfrax on top of the no-blow to fix an unstoppable leak on a O320 cylinder. The material is good for about 1500F and compresses and seals well.

Available on Amazon - used by the ceramic pottery folks in their kilns. I used it a lot at the steel company I used to work at (a lifetime ago). About 20 bucks for 18in x 12in piece.

Ron
 
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