CO in the cockpit

NealRomeoGolf said:
half the Pipers, mine included, called a Cherokee
Click to expand...
I used to call ATC with "Archer 76V" or "Warrior 13M" and they ALWAYS just call me back as Cherokee. Whatever. If I rent a single engine now it's an actual 1961 PA-28 CHEROKEE, makes my flying experience just a bit more harmonious

It was interesting that although every Aztec came as PA-23 from Piper ATC calls the Aztec PA-27.. something I learned in the last 6 months. Apparently the Apache and Aztec have a wide enough performance margin that the distinction PA-23 vs PA-27 for ATC reasons is warranted. I once had ATC ask if the SR-22 I was flying (SR22 slant golf is what I always said) was turbo.. when I replied yes they shot me up to 17K. Was pretty cool!
 
A Martin said:
Unrelated trivia ..... many years ago the auto industry solved the problem by slightly pressurizing the cabin with fresh incoming air so that nothing could enter via poor seals or leaks ... they do it by running the heater-AC fan at a very low setting even though the fan is switched off... easy to do in a car because new air enters from the cowl area just under the windshield ....

I am not familiar with Cessna incoming air source ... but if you are certain no gases enter there I would just leave the fan at lowest setting for best air quality

Here is the thing .... a moving aircraft or auto creates a slight vacuum in the cabin ... thus replacement air tries to come in wherever it can ... a low fan pushing fresh air halts all the other sources.

Just like if you are driving your car and crack a window just a bit is the worst thing because the cabin vacuum rises significantly . Sounds counter-intuitive but true .... using slightly pressurized fresh air causes cabin air to exit thru old seals and leaks ... and that is ideal
Click to expand...

Unfortunately, unless on the interstates, I drive with a back window cracked about 1/2 inch. Why? Because these new cars are so bloody quiet, I can't hear the emergency sirens.
 
Resurrecting this thread instead of starting a new one.
I have the Forensics CO detector. It's relatively new at about 2 months old, and calibrate it before starting the engine.

My situation is a bit opposite of @Mtns2Skies.
I do get a CO reading of about 38-40ppm on climb out, but only with cabin heat on.
In cruise, it's about 9-14ppm, again, only with cabin heat on.
Readings are consistently 0 with cabin heat off.
Nothing on ground really has been noticed, heat on or off.

I have removed the shroud and visually inspected, bore-scoped it very well inside, checked pipes, EGT probe, etc., and cannot find anything obvious.
Have read through some of the other threads too, and at least one post indicates scratches could be cracks. I do see some small scratches, but I would think some scratches are likely, since we removed the shroud which was a bit of a contortion.
Will pressurize it next time I have the cowling off and soap it up to check for cracks.
I know this likely indicates a small muffler crack/leak. I've accepted it, budgeted for it, added it to the list of other leaks, set up appointment with therapist, will repair, and soon move on.

With that said, I'm wondering why the higher reading during climb vs. cruise, which seems to be about the same RPM. (2350-2450)

The only theory I can muster, is that I know the main air source to the cabin heat (for my plane) is the hole in one of the front engine baffles (as @Dan Thomas referred to in post #38)..
During climb, I've got approximately same RPM's as cruise, however, less speed and different angle toward wind, providing less airflow through the shroud, thus less fresh air to mix with CO coming out of suspected crack, hence the higher reading.

Sound legit?
Any crack in that theory (besides the one mentioned)?

*awaits eman1200 crack about me being on crack.....
 
Skyrys62 said:
Resurrecting this thread instead of starting a new one.
I have the Forensics CO detector. It's relatively new at about 2 months old, and calibrate it before starting the engine.

My situation is a bit opposite of @Mtns2Skies.
I do get a CO reading of about 38-40ppm on climb out, but only with cabin heat on.
In cruise, it's about 9-14ppm, again, only with cabin heat on.
Readings are consistently 0 with cabin heat off.
Nothing on ground really has been noticed, heat on or off.

I have removed the shroud and visually inspected, bore-scoped it very well inside, checked pipes, EGT probe, etc., and cannot find anything obvious.
Have read through some of the other threads too, and at least one post indicates scratches could be cracks. I do see some small scratches, but I would think some scratches are likely, since we removed the shroud which was a bit of a contortion.
Will pressurize it next time I have the cowling off and soap it up to check for cracks.
I know this likely indicates a small muffler crack/leak. I've accepted it, budgeted for it, added it to the list of other leaks, set up appointment with therapist, will repair, and soon move on.

With that said, I'm wondering why the higher reading during climb vs. cruise, which seems to be about the same RPM. (2350-2450)

The only theory I can muster, is that I know the main air source to the cabin heat (for my plane) is the hole in one of the front engine baffles (as @Dan Thomas referred to in post #38)..
During climb, I've got approximately same RPM's as cruise, however, less speed and different angle toward wind, providing less airflow through the shroud, thus less fresh air to mix with CO coming out of suspected crack, hence the higher reading.

Sound legit?
Any crack in that theory (besides the one mentioned)?

*awaits eman1200 crack about me being on crack.....
Click to expand...
You're also at full power and very rich in the climb. The engine is burning more fuel and making more exhaust gasses, and those gasses will be richer in CO.

CO is the product of insufficient oxygen during combustion. An engine running lean of peak produces little CO and more CO2 because the limiting factor in the reaction is fuel. The richer the mixture gets, the more CO is created.
 
I had something similar. I took the muffler off and put rubber stoppers in all the tube ends. The last one had a hole in it, in which I put a fitting. I ran a little pressure in it and painted it with soapy water. A weld on the end of the muffler was cracked. It was almost invisible without the pressurization.

When you are in the climb, the extraction of air around the engine may be diminished. At cruise, you should have a nice flow through the front, down across the cylinder fins and out the bottom near the firewall.
 
Mtns2Skies said:
My CO detector tells me I've got about 40 PPM in the cockpit during climb/cruise. If I turn air or heat on it will drop significantly. I have all new window/door seals and my plane is equipped with anti-CO scoops to pressurize the tail and prevent CO from entering from the tailcone.

So where should I be checking? Firewall leaks?
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What makes you think that these "scoops" aren't putting CO into the tail cone? The cabin being the fat part of the aircraft shape is the low pressure area. The airflow is often into any vent in the tailcone (scoops, vents, elevator carry through, etc...) and exits the fuselage around the cabin doors/windows.
 
Jim K said:
You're also at full power and very rich in the climb. The engine is burning more fuel and making more exhaust gasses, and those gasses will be richer in CO.

CO is the product of insufficient oxygen during combustion. An engine running lean of peak produces little CO and more CO2 because the limiting factor in the reaction is fuel. The richer the mixture gets, the more CO is created.
Click to expand...

So a combination punch.

How exhausting..
upload_2022-2-20_18-11-55.png

edit: but I guess I shouldn't be complaining to you right now lol
 
A generic diagram, but the chemistry is the chemistry... The big difference between a car engine and the traditional aircraft engine is the point at which the unburned hydrocarbon curve turns up on the right (lean) side.
Green line is purd near "Peak". You can see how fast the CO goes up as you get rich.

airfuel.jpg
 
LesGawlik said:
I had something similar. I took the muffler off and put rubber stoppers in all the tube ends. The last one had a hole in it, in which I put a fitting. I ran a little pressure in it and painted it with soapy water. A weld on the end of the muffler was cracked. It was almost invisible without the pressurization.
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In Canada we have a blanket AD that applies to all aircraft using the exhaust system as a source of cabin heating: https://wwwapps.tc.gc.ca/Saf-Sec-Sur/2/cawis-swimn/AD_html.aspx?ad=CF-90-03&cn=CF&l=E

It must be done every year or 150 hours, whichever comes first. An excerpt from it:

  1. Remove the heater jacket (when removable) and inspect the outer surface of the inner core (exhaust pipe) for signs of combustion products.
  2. Thoroughly clean the exhaust pipe of all dirt and corrosion.
  3. Carefully inspect the exhaust pipe, using a hand-held magnifying glass and light/mirror, for signs of cracks and pinholes. Pay particular attention to welds, seams, and their adjacent areas.
  4. Any suspect areas are to be further evaluated by a pressure test or other means recommended by the aircraft manufacturer.
  5. Repair or replace, before further flight, exhaust type heat exchangers containing cracks or holes in the exhaust pipe.
Note that number 4. Pressure testing suspect areas. We got so we did the pressure test every time anyway because the visual so often didn't catch tiny cracks. But one must use minimal pressure so that bubbles will form rather than the soap being blown away. While we were at it we would soap the exhaust risers as well, often finding more serious stuff and leaking exhaust gaskets that lead to erosion and destruction of the cylinder head's exhaust port face. The test can be done with the system in the airplane but the sparkplugs must be in or the pressure just escapes out an open intake valve.

CO leakage is a serious deal. In colder Canada that heating system is used frequently, and the government got fed up with too many fatal accidents due to leaking exhaust systems. And a tiny crack eventually becomes a big crack or a total failure that can set the airplane afire. Fun, eh?
 
I don't think that's a big deal to do. I think if you get a Power Flow they want that pressurization test every year as a condition of continued airworthiness.
 
I use a Leaf Blower to pressurize in place and have found quite a few components that were leaking UNLESS some document requires a different method. Suggest
a re-read of Dan’s post. Gaskets cost way less than cylinders. Bubbles will mark leaks far better than by visual means.

On one aircraft all seemed well but the Leaf Blower and soap and water showed otherwise. What looked like a scratch turned out to be cracked about 400degrees around the circumference. Being attached to the cylinders is what held it together.

On Lycoming 172s there is a “ horse shoe “ doubler that is spot welded to the pressure can. When ( not IF) the weld fails you cannot find it visually as it is sandwiched between 2 pieces. The aircraft would not make it around the pattern w/o turning the Detector Dot black.
 
Article if anyone is interested. https://www.cessnaflyer.org/mainten...xhaust-system-101-inspection-maintenance.html

This reflects what @Dan Thomas said above:
1. Always consult the aircraft maintenance manual to ensure that the mufflers are pressure-tested to the required point. Some only need 2-3 psi and some as high as 15 psi.

Which leads me to a question: How much pressure does a leaf blower or shop vac put out? I know it would depend on horsepower, etc., but is there a range anyone knows of?

Also thought this was interesting. Never heard it before..
• Pencils shouldn’t be used to mark exhaust components during maintenance because the graphite can weaken the metal as the exhaust heats up in use and cause a crack.
 
An earlier revision of that Canadian AD I posted recommended the use of a vacuum cleaner' air output. That's very little pressure, less than 1 PSI typically. They changed that to "a method recommended by the manufacturer." And the use of a vacuum cleaner presents the risk of blowing contaminants from the vacuum hose into the cylinders. For a long time I had an old vacuum dedicated to that job alone. It was thoroughly cleaned out and the hose was washed out until there was nothing in it at all. Later I used a small air pressure regulator and set it by soaping a known leakage point such as where the riser is clamped to the muffler, and made it produce nice big bubbles. Like I said, too much pressure just blows the soap away and you see nothing. And the soapy water needs to be really soapy, like 1/4 dish soap and the rest water. Rinse it all off when you're done; soap is alkaline and corrosive. And it stinks.
 
My comment was I use the Leaf Blower “unless some document requires a different method”. I also use the method Dan described using a “ known leakage point” to “calibrate”. If an AD mandates using the Mfg . Methods then that’s how it’s done.

Things I like about about the Leaf Blower are it’s easily portable, connection is readily accomplished and it’s an EFFECTIVE way of finding defects. Disassembly and pressure testing is a good thing but may not be done when needed. I suggest
folks have this done in October regardless of calendar time or hours flown.
With rentals there is always the possibility that some clown hit OFf during a mag check and then went back ON. The first thing the Intrepid Birdman does is see if there were witnesses.

This “ minimally invasive “ method can also be a “ first check” of the Induction System.
Connect at Carb Heat Duct , carb Heat ON and Throttle OPEN. Even at a low pressure you may find leaking hoses and gaskets.
 
I’m convinced there is one person alive BECAUSE of a cylinder failure.
Lycoming 172 landed with a very rough engine and power loss. One jug was split open. ( AD) However that was a GOOD thing! Lycoming 172s should have a “bumper” installed in the Vacuum Pump area that accepts air loads that would push the lower cowl aft. This aircraft didn’t have it! This allowed the lower cowl to crunch the airbox and wear a hole in the muffler.

This took place in winter so I’m sure cabin heat would be ON ; with a hole in the muffler! Even crazier than the crappy maintenance was finding the fresh air vent
Inlets in the wing leading edge were duct taped over.
 
Magman said:
I’m convinced there is one person alive BECAUSE of a cylinder failure.
Lycoming 172 landed with a very rough engine and power loss. One jug was split open. ( AD) However that was a GOOD thing! Lycoming 172s should have a “bumper” installed in the Vacuum Pump area that accepts air loads that would push the lower cowl aft. This aircraft didn’t have it! This allowed the lower cowl to crunch the airbox and wear a hole in the muffler.

This took place in winter so I’m sure cabin heat would be ON ; with a hole in the muffler! Even crazier than the crappy maintenance was finding the fresh air vent
Inlets in the wing leading edge were duct taped over.
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I've seen those bumper missing, but they're on the starter bracket and contact a shockmount on the landing light housing. They keep the airloads and pressure differential in the cowl from pushing the cowl upward.

The airbox commonly gets chafed by the cowl. Sometimes some minor mods need to be done to get the thing centered in the cowl opening, or even off toward the pilot's side a bit to allow for the engine's torque reaction to the propeller, which drives the airbox to the starboard side and whacks it against the cowl. Those airboxes are fearsomely expensive, too. Last I checked Cessna wanted over $4K for one.

I knew the owner of a Cherokee, recently bought, whose mechanic went to do the first annual and did the cabin heat AD. Took off the shroud around the muffler and found a hole he could stick his fist through. Logs showed that the AD had been performed for years, most recently a year and 40 hours before. Yeah, sure it was. They took the muffler off and it fell apart in two pieces. If the owner had flown it in the winter it would almost certainly have killed him. "No wonder I was getting a headache when I flew," he said.

Taping off the fresh air ducts is illegal, but people do it because the Cessna wing root vents are so leaky. Another POA thread addresses that.
 
Do Lycomings have the vacuum pump in front? Well; Dan is correct and it’s the Starter area. A few150s use something similar. I apologize for “ sleep writing”.

The poor pilot of the rental had about a 2hour flight over the Appalachians in the winter ahead of him. The condition of the aircraft would be a NTSB topic if the jug hadn’t failed.

The owner was livid when I told him the Airbox and Muffler also need replacement.
I took pix of the mess and walked. The owner then sent a Tech to rescue his junk.
That didn’t seem like a happy situation either.

My advice is for rental pilots to carry some type of CO detector. Even the little dot just could save your life.
 
Magman said:
Do Lycomings have the vacuum pump in front? Well; Dan is correct and it’s the Starter area. A few150s use something similar. I apologize for “ sleep writing”.
Click to expand...
Lyc pump on the back of the engine. The 150's O-200 has it under the nose of the engine.
 
Crack verified, going to replace muffler.
Some vendors offer two options. With or without internal baffles/cones.

Current one was purchased without baffles, and installed approx 720 hrs ago.
Oddly, I can get one with baffles for less money.

I really hate to have the baffles to worry about, but I like saving money, and there is some talk about proper back pressure, etc.
Not sure I'd save money if the baffles disintegrate prior to the other parts failing.

Thoughts?
 
Type aircraft?
It seems at least 20+ years since baffles were eliminated of on replacement mufflers for Lycoming 172s. I think thickness was increased on some components at that time.
If legal; I would prefer no baffles
 
172N
Just seems odd that some suppliers/vendors would only offer the one option of having them. This is a U.S. based company, Nicrocraft.
Acorn is Canada based, and offers both options, but at a somewhat higher price.

Edit:
I called them back today, and apparently the representative was misinformed.
The person I spoke with today, said the same thing as @Magman .. they eliminated the baffles years ago.
With that said, it is funny to me that an A&P who looked at my plane, went just nuts over there not being any baffles in my current muffler, and said it was illegal.

So now that mystery is over.
The next question would be is there any real difference in quality between the suppliers.
Knisely
Acorn
Nicrocraft
AWI
Dawley (now Tailwinds?)
(others)

Anyone have any experiences or opinions to share?

One thing I've noticed, is that Knisely does not include holes in the gusset where the tailpipe is welded to the muffler. This was a concern to me, as it seems like a hot spot due to no real air circulation, and is where my existing crack was.
Some of the other suppliers now include drilled holes in that area.

I would just drill holes, but do not want to void the warranty.
 
Last edited:
Skyrys62 said:
172N
The person I spoke with today, said the same thing as @Magman .. they eliminated the baffles years ago.
With that said, it is funny to me that an A&P who looked at my plane, went just nuts over there not being any baffles in my current muffler, and said it was illegal.

One thing I've noticed, is that Knisely does not include holes in the gusset where the tailpipe is welded to the muffler. This was a concern to me, as it seems like a hot spot due to no real air circulation, and is where my existing crack was.
Some of the other suppliers now include drilled holes in that area.
Click to expand...
The internal baffles were more hassle than they were worth. They weren't even proper baffles. They were tubes with big holes in them that ran across the muffler between pairs of inlets, so that inlet gasses had to push their way through them. The proper thing is that the pulse of exhaust enters the muffler with no restriction, and passes OUT through the baffling. That levels the pulses out to cut the noise.

The baffles got a lot hotter than the rest of the muffler, so they expanded more and pushed against the end walls, flexing the walls and causing cracks. Eventually they just buckled and broke apart and rattled around in there, damaging the can until they finally fell out the tailpipe. Worse than useless.

That area around the tailpipe weld is exposed to ambient cooling airflow. We never had serious problems with those cracking. We did have trouble with the horseshoe-shaped baffle around it, since the muffler can gets hotter than the horseshoe, stressing and causing causing failure of the spot welds, sometimes cracking the can in doing so. Differential expansion is a problem all over the place with exhaust system components.
 
Called Knisely directly (not the distributor), and they drill the holes on the inside of the tailpipe in that area. Their reasoning was that the 'patch' for reinforcement was welded all the way around, and created a sealed void between the two pieces of metal, causing heat and pressure build up.

They also glass bead blast their finished product.
Probably no real effect on thickness or anything. Just wondered why they would take the time, other than to pretty it up. They are 321 stainless like the other offerings for this model.
 
I have the smell of fumes during slow flight phases. How do resolve this problem?



Mtns2Skies said:
My CO detector tells me I've got about 40 PPM in the cockpit during climb/cruise. If I turn air or heat on it will drop significantly. I have all new window/door seals and my plane is equipped with anti-CO scoops to pressurize the tail and prevent CO from entering from the tailcone.

So where should I be checking? Firewall leaks?
Click to expand...
 
I used Knisley parts for a complete replacement of the exhaust system on my Piper Warrior II (-161). I did all the work under A&P-IA supervision. Knisley responded quickly answering all my questions and were quite accessible for all my inquiries. I spoke with one of the Knisleys a couple of times. Everything fit perfectly.

I would not hesitate to use Knisley again.
 
Ordered one yesterday.
They were indeed great to work with. Very responsive and informative.
I went through The Parts Exchange, (who I think may be owned by Knisely) ...also great folks to work with and great pricing.
https://www.aircraftexhaust.com/
 
So the A&P and I are discussing torque values during the muffler install.

C172N model

For the risers to cylinder connection, the table of limits says 204 in lbs (min.) for 5/16 x 18 nuts.
After some research, there seems to be concern about that figure.
I see others recommending to never use that much, as it may pull the studs out.
Others say 160-180 and yet others 100, etc.
This is mostly sifting through multiple posts in Mooneyspace, Vans forums, etc.
He says we'll just do with 140, and check/re-tighten after first flight.

Also, the clamps that hold the riser to the muffler.
Those are 1/4 x 20
Table of limits says 96 in lbs.

Edit:
Called Western Skyways, who does install mufflers according to them, and they use 80-90 in lbs on the exhaust nuts. 40-50 in lbs on the clamps.

Called Lycoming, and they say 204 in lbs on the exhaust nuts.

bizarre..

What do you installers use?
 
Last edited:
Skyrys62 said:
So the A&P and I are discussing torque values during the muffler install.

C172N model

For the risers to cylinder connection, the table of limits says 204 in lbs (min.) for 5/16 x 18 nuts.
After some research, there seems to be concern about that figure.
I see others recommending to never use that much, as it may pull the studs out.
Others say 160-180 and yet others 100, etc.
This is mostly sifting through multiple posts in Mooneyspace, Vans forums, etc.
He says we'll just do with 140, and check/re-tighten after first flight.

Also, the clamps that hold the riser to the muffler.
Those are 1/4 x 20
Table of limits says 96 in lbs.
We tightened them until they look like they are done, almost bending, and the clamps are touching, but never hit 96 in lbs.

Anyone know?.. or have practices that are proven?
Click to expand...
If you pull the studs out with specified torques, you have worse problems. Overtightening will usually bend the riser's flange and sometimes start cracking.
And I have never seen 1/4" bolts in those clamps, and certainly not coarse-threaded bolts. Always AN3 bolts; #10-32 thread.

upload_2022-3-8_11-25-15.png

upload_2022-3-8_11-26-59.png
 
Dan Thomas said:
If you pull the studs out with specified torques, you have worse problems.
Click to expand...

Which specified torque is correct, being the question.
I don't argue any of it. It's just bizarre to me how many different answers there are, and yet to see any concrete documentation.


Dan Thomas said:
And I have never seen 1/4" bolts in those clamps, and certainly not coarse-threaded bolts
Click to expand...
That's what was installed. Guess I'll be ordering.
 
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Skyrys62 said:
Which specified torque is correct, being the question.
I don't argue any of it. It's just bizarre to me how many different answers there are, and yet to see any concrete documentation.
Click to expand...
Two torque values. The first is for the stud as it's screwed into the head. Its thread is a bit oversize so it has to be forced in. It's like that so it doesn't back out when you remove the nut to get the exhaust off.

upload_2022-3-8_16-12-49.png

The nuts are 5/16" and have no special torque, so you use the standard torque:

upload_2022-3-8_16-14-41.png

All from the Lycoming Direct Drive Overhaul Manual.
 
Skyrys62 said:
This is mostly sifting through multiple posts in Mooneyspace, Vans forums, etc.
He says we'll just do with 140, and check/re-tighten after first flight.

Also, the clamps that hold the riser to the muffler.
Those are 1/4 x 20
Table of limits says 96 in lbs.

Edit:
Called Western Skyways, who does install mufflers according to them, and they use 80-90 in lbs on the exhaust nuts. 40-50 in lbs on the clamps.

Called Lycoming, and they say 204 in lbs on the exhaust nuts.

bizarre..

What do you installers use?
Click to expand...

Your mechanic has convinced you to deviate from the manufacturer's recommendation based on SGOTI. Do you really think that's a good idea?
 
3393RP said:
Your mechanic has convinced you to deviate from the manufacturer's recommendation based on SGOTI. Do you really think that's a good idea?
Click to expand...

I wouldn't be spending time calling multiple manufacturers/installers and asking installers/mechanics here, if he has convinced me.. or if I thought it was a good idea. I want to know for sure.
Can you help?
 
The regulations say this:

upload_2022-3-9_10-12-37.png

Now. What part of that do mechanics not understand? They are supposed to have those manuals and they're supposed to be up-to-date. I can easily find many of them as .pdf's on the 'net, for free, for reference when answering questions here. Some of the stuff is free from the OEM, parts catalogs being examples.

The torque stuff I posted was straight out of the Lycoming DD overhaul manual. Those numbers will also be in the Cessna maintenance manuals. This isn't rocket science, but if one just makes up his own stuff, and I've seen way too much evidence of that and we hear it all the time on POA, he's risking failures.
 
Skyrys62 said:
I wouldn't be spending time calling multiple manufacturers/installers and asking installers/mechanics here, if he has convinced me.. or if I thought it was a good idea. I want to know for sure.
Can you help?
Click to expand...

I would follow the manufacturer's information and torque the fasteners to 204 and 96 inch pounds respectively. It won't overstress the fasteners, and due to heat cycling of the exhaust, that torque is required.

If the nuts loosen, it will probably blow out the exhaust gaskets, and your problem will return.
 
Dan Thomas said:
Now. What part of that do mechanics not understand?
Click to expand...

That's what is so bizarre to me.
It isn't like this hasn't been being done for decades. And when you start asking other mechanics, and especially installation shops, and you get differences like 80-90 vs 204, it's really hard to understand.

I originally came to the 204 in lb conclusion since I could find no other documentation mentioning the exhaust nuts specifically. But while looking for that documentation, came across various warnings, suggestions, methods used, etc., by others.
Some even saying they contacted Lycoming, Vetterman Exhaust, and still have different numbers than the 204 in lbs.

The scary warning was that 204 in lbs will pull out the studs when the engine gets warm due to expansion. After that, I wanted make certain I wasn't missing something, so I started making my own calls, etc., to get some concrete info.
Besides others indicating it has happened, the fact that the studs are put in at 40 in lbs, seems light when comparing to 204 in lbs and/or more when it's warmed up. I know the engineers have it all figured out, but again, just another reason to ensure I have the correct information before we proceed.

Western Skyways said 80-90 in lbs
Lycoming said 204 in lbs.

I'll elect to go with Lycoming, but I do know they sell cylinders ;)

(that's a joke)
 
3393RP said:
I would follow the manufacturer's information and torque the fasteners to 204 and 96 inch pounds respectively. It won't overstress the fasteners, and due to heat cycling of the exhaust, that torque is required.

If the nuts loosen, it will probably blow out the exhaust gaskets, and your problem will return.
Click to expand...
The AN3 nuts on the clamp bolts should get very little torque. From the Cessna 172 manual:

upload_2022-3-9_13-6-4.png

upload_2022-3-9_13-7-27.png
Just tighten them to get the clamp tight on the connection.
 
Skyrys62 said:
That's what is so bizarre to me.
Western Skyways said 80-90 in lbs
Lycoming said 204 in lbs.
Click to expand...
Well, now, the exhaust system is an airframe system, not an engine system, so we should see what Cessna says. Their engine section discusses the exhaust system, but gives no torques for the hardware, so we revert to the standard torques they give in the General section of the manual:

upload_2022-3-9_15-38-42.png

The 5/16-18 nuts on that system should get 80-90 inch-pounds. That would be where Skyways got their numbers, and they would be much closer to correct than the 204 I dug out of the Lyc manual. You will be really surprised at how little torque that is. Be sure to use new lockwashers.
 
Dan Thomas said:
Their engine section discusses the exhaust system, but gives no torques for the hardware
Click to expand...

Exactly what I kept finding...nothing.
And find it odd Lycoming wouldn't say or even know that it's an airframe part?...plus be so adamant that it was 204 in lbs.
He actually got a little irritated when I was asking why Western Skyways uses a much lower number.

Still in disbelief that this is done so many times per year and such an unknown to so many of the folks responsible to know.

And even 200 in lbs doesn't feel like much to me. 80 feels like a joke.
 
Skyrys62 said:
And even 200 in lbs doesn't feel like much to me. 80 feels like a joke.
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And that's aircraft maintenance. These are not tractors. The hardware is small, the components are light and easily distorted. With fine threads you really have to be careful. Those AN4 1/4" bolts that hold your brake calipers together get no more than 70 inch-pounds. I have found them stretched, with the threads necked, and had one fail on an airplane due to overtorquing.

The most-used tooling on a light airplane is a 1/4" socket set. 3/8" drive occasionally, mostly on engine or prop mounting bolts. 1/2" drive really rarely, and then usually only on the big landing gear hardware like the inboard gear leg bolt on the 172s and 182s with the rod gear.
 
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