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rhtrudder

Leaky exhaust

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Had a yellow stain appearing from around the left side of the cowl , looks like it was coming out of the exhaust where it fits into the head , could it be that the pipe wasn’t seated properly and maybe allowing unburnt fuel to leak out hence the yellow colour, I’ve had a go at reseating the compression ring but haven’t flown yet , has anyone come across this. Greg 

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Rotax 912??

 

If it is exhaust gas its unlikely to be unburnt fuel more likely to be exhaust/heat staining.

 

If it is exhaust gas -  I implore you to be very very careful - potential hazards -  fire and carbon monoxide poisoning. In addition, engine performance may be negatively impacted.

 

I always re seat my exhaust system with generous amounts of anti seize past "buttered" on to the mating surfaces - this facilitates a good seat BUT may make a bit of a mess on first engine test run up, with anti size spattering around the inside of the cowl and down the belly of the aircraft.

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Anti-seize?????

Why don't you use high temperature gasket sealant? I use it on the joints on my bike's exhaust, even where the exhaust pipe slips into the cylinder.

image.png

 

Admittedly, this engine does not have an exhaust manifold, but the sealant has not been weakened by the heat it is exposed to.

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914 rear left , the stain looks like it starts at the exhaust port , it’s on the carby , firewall , oil tank and streaming away outside from the cowl , the only thing that would dissolve it was carby cleaner in a spray can .

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19 minutes ago, rhtrudder said:

914 rear left , the stain looks like it starts at the exhaust port , it’s on the carby , firewall , oil tank and streaming away outside from the cowl , the only thing that would dissolve it was carby cleaner in a spray can .

95 fuel leaves a yellow / very light brown stain.  I mainly use Caltex and some times Shell. Cheers

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They are spherical seated held by springs to allow the system to flex. The surface must be damaged or distorted. Rework or replace the affected part. Nev

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Normally a high temp silicone like sealant is used on exhaust ports that have some form of seal or gasket. I think it's the RED one. That's what I used on the Continental There must be something out of shape .It's pretty important to seal it as a leak will probably get worse and damage the seating area. as well as the flame and CO risk.  Don't over tension anything. Nev

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If you have a serious exhaust leak for more than a short time, be aware that high temperature exhaust gases being propelled through the gap, will create erosion of the mating surfaces, thereby making a good neat fit of the mating parts, impossible in the future.

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There was a mark on the ring which I managed to work away with an abrasive tape and reassembled with permatex exhaust goo , but I may be up for a new pipe, do you think the yellow stain would be from the leak , blowing a gale here at the moment so reluctant to go flying 

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I think Skippy is correct with the high temp anti sieze, although I just use this on the "ball" joints to the muffler and leave the head end dry. 

You can buy the pipe end new and just cut the old one off and weld on the new one.

 

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1 hour ago, Downunder said:

I think Skippy is correct with the high temp anti sieze, although I just use this on the "ball" joints to the muffler and leave the head end dry. 

You can buy the pipe end new and just cut the old one off and weld on the new one.

 

From memory, the anti seize is recommended in the Rotax 912 maintenance manual.

 

I find  the anti seize to be useful when assembling all parts of the exhaust system (not just the ball joints) I even use it on the head exhaust studs  - it just helps seat everything in its correct position without binding/galling and the residue (left after a run up) facilitates the small expansion/contraction movements of the ball/socket joints.

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Don't anti-seize and high temperature silicone do different jobs?

 

The goal of anti-seize is to prevent threads in bolts from "rusting on" to the threads of their nuts. The use of anti-seize is aimed at enabling the unscrewing of the nut if the components held by the nut have to be disassembled. One place that anti seize is useful is on the threads of spark plugs.  If the desire is to prevent the nut from undoing, then a thread lock solution is used.

 

Silicone sealants so just that they are called - seal gaps. The gaps might be microscopic, as between the faces of tow machined parts, or macroscopic as in the case of sealing an exhaust pipe leak.

 

Anti-seize won't do the best job of permanently sealing a gap. In fact, it might be better to use a muffler sealant like this one https://441py33rout1ptjxn2lupv31-wpengine.netdna-ssl.com/wp-content/uploads/tech_docs/tds/80335.pdf

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I use anti seize between the ball joiners of the metal muffler.  rotax call for loctite anti seize.  On  my 912 ULS I have the stainless steel CKT pipes and muffler and use Kopper Kote anti seize (It is not a silicone sealant) between the stainless steel to stainless steel areas. It is applied every 100 hrs and just lubes for the movement of the exhaust in operation.   I do not use anti seize between the pipes and the alloy cylinder heads.  They are a clean dry fitup.   I'd expect that for any marks at the heads seating area you would need to use a perfect shaped header pipe and lap the fit under advice from Rotax.  You should not use a silicone paste there just get the faces mated perfect.

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7 hours ago, Blueadventures said:

just lubes for the movement of the exhaust in operation. 

The OP did not say anything about the fact that the manufacturer allowed for movement as a normal condition. Therefore, I can see why anti-seize is specified by the manufacturer. In that case, don't use silicone sealant (although it is somewhat flexible after it has set. 

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Of course anti seize & silicon (all sorts) do different "jobs".

 

The anti seize I use on my 912 (specified by Rotax) has a very high metal content (as many do) carried in a lubricant. The metal fraction remains even after heat/time has removed the carrier lubricant. The metal particle themselves have a degree of lubricity/slip that is beneficial in situations where a small degree of movement between surfaces (eg exhaust ball/socket joints) is expected. It also helps with initial set up/alignment where binding & galling of contact surfaces may make settling into position difficult.

 

Despite routine partial or full removal/reassembly of my 912 exhaust system, I have never had an exhaust leak problem.

 

As you point out silicon sealants are for gap sealing - whole new subject.

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