Continental O-470K, loosing compression on number 4 cylinder

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Skylark60
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Continental O-470K, loosing compression on number 4 cylinder

Post by Skylark60 »

Last year at annual inspection I replaced #4 jug due to burnt valve

This year at annual the same cylinder location is showing 40 psi with only 25 hrs of service

I have an egt probe on that cylinder and keep the temp under 1325 F.
The blow by is around the rings and not the valves

Q: Could this be stuck rings or might the rings rotated and aligned the gaps?

Can a cylinder fail this fast?
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ahramin
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by ahramin »

1. Are you following Continental's Maintenance Manual for compression testing and boroscoping cylinders?

2. Was the Continental SB for cylinder break in followed when you installed the new cylinder? Improper break in would be my first guess for low compression on a new cylinder.

3. You state that you are keeping the EGT under 1325. Where did you find this limitation?
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Skylark60
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by Skylark60 »

I ran break in oil throughout the 25hrs
I flew across Canada, AB - NL about 20hrs

I do not know for sure what the upper limit for the EGT so I keep it below 1325 F when possible (other than takeoff it can go to 1360 for a short period of time). I try to keep it below 1300 in cruise

Is the cylinder toast? Or can it be just stuck rings?

I am stumped
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ahramin
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by ahramin »

Is the cylinder toast? Maybe. Depends on whether or not you are following Continental's Maintenance Manual for compression testing and boroscoping cylinders. It has appropriate metrics for deciding whether a cylinder should be removed.

Is that the only part of the break in procedure that you followed? Mineral oil for 25 hours? None of the rest of the procedure? If that's the case you may have glazed the cylinder. If so, it doesn't require a new cylinder but will require removal and honing. What does the boroscope show on the cylinder walls? What power settings did you cruise at in your trip across Canada?

Why do you think there is an upper limit on EGT? Where are you getting this information?
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by photofly »

There’s no limit on EGT. The exact value of EGT depends very much on where the probe is mounted. EGT is used for leaning, you watch it go up and down as you move the mixture knob, but you don’t care about the number.

CHT is the value you care about: keep it at or below 350 degrees F if you can.
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2R
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by 2R »

What power settings are you using ?
55 , 65 , 75 percent ?
75 percent is recommended for break in . Anything less and it is not gonna break in . Red lines are objectives not limitations in a break in period :)
Once firmly broken in running the 65 numbers should get to TBO and beyond if you avoid shock cooling on top and descent.
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by Big Pistons Forever »

My 02 cents

For the carburated O 470 an EGT gauge is a waste of money. The engine does not have even enough fuel distribution to lean by reference to the EGT. Just leaning until the engine starts to run rough then enriching until it is smooth again works fine and you can't hurt the engine.

Where you most definitely can hurt the engine is by over heating the cylinders. A multi probe CHT that shows the CHT on every cylinder is highly recommended. Ideally max CHT should not exceed 380 F and if it is at 400 you need to do something now, usually this happens in a climb and so you need to increase airspeed. If it is in cruise crack a bit of cowl flap.

Always have the cowl flaps full open on the ground and in the climb.

As a general comment my experience with O 470's is that the cylinders all tend to quickly lose compression after overhaul and then stabilize in the low 70's or mid to high 60"s for the life of the engine

With respect to your cylinder the key issue with differential pressure checks is where the air is bypassing. If it is coming through the exhaust pipe thenit is going through the exhaust valve. This should be investigated by means of a boroscope to look for exhaust valve distress. Failure of the exhaust valve can easily result in total engine failure.

Air passing out the crankcase breather means the air is going through the piston rings. This is much less concerning. The only significant effect will be increased oil consumption and possible fouling of the spark plugs. If the plugs are OK then I would suggest doing nothing and fly another 100 hours and see what happens. It is entirely possible that the compression test will come up with you doing nothing.

As was postulated if the cylinder stays with low compression then it is probable that the cylinder walls are glazed due to poor break in procedure. However again there is no reason to pull a cylinder with a 40/80 DP reading unless there are other issues.

Finally the first 4 hours or so will pardon the pun. make or break your break in procedure. After a cylinder change, keep ground running to a minimum and make the first flight at least 2 hours long and run the engine hard with 75 % power. You will initiallyu see high CHT's on the affected cylinder but after usually about an hour or so there will be noticeable drop in CHT's. This indicates that the rings are starting to seat properly and the critical part of the break in has happened.
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Skylark60
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by Skylark60 »

Thanks for this

I hear you on the egt, it is the only reference I currently have to temp and I have been using 1300 as a guideline.

I flew 2500 squared for the first several hours but then had to transit the country. Much of this flight was 2300 squared due to fuel capacity and distances to be covered.

I think I did glaze the jug whereas the compression is leaking around the rings.

The AME just said the cylinder had to be replaced, he did not use a bore-scope.

Am I correct to assume that a good honing and ring replacement should remedy the low compression?
That would be so much easier.

Thanks for your two cents worth, I believe it will save me a load of dollars.
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by 2R »

Learned something again , hope I do not forget it again :)

The old Alabama valve cleaner will not work if it is glazed .
Could you get the new jug honed under warranty ?
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by rigpiggy »

Ask if he has followed continental sb03-3..... read this https://www.aopa.org/news-and-media/all ... -borescope
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by rigpiggy »

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hangarline
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by hangarline »

Am I correct to assume that a good honing and ring replacement should remedy the low compression?
That would be so much easier.
It is quite possible that will remedy your compression, but don't mistake less expensive for being"so much easier". It still takes just as much work to remove the cylinder and hone it, as it does to replace it.
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Skylark60
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by Skylark60 »

Thank you all, great information.

I will pick up a borescope so I do not have to depend upon a poorly equipped AME.

I will repost when resolved
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Piston Power
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by Piston Power »

Another possibility... Incorrectly staggered piston rings at installation could also cause your low compression. Having said that, if your AME is experienced, it is very unlikely.
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by iflyforpie »

Sounds like glazed cylinders to me.

Piston rings rotate. The main reason for staggering them is to help with the break-in process but every time I've pulled a jug the gaps have been random. This is why a cylinder leak down is mainly for trend monitoring and you'll see numbers go up and down. It's also done at a much lower pressure over a much longer time than a combustion event.

But you want to show your engine absolutely no mercy during break in. You need to seat those rings and high pressure and temperature is the only way to do it. It's not the time to learn how to taxi your airplane or go on long cross countries where you need to save fuel.

You probably don't need to replace the cylinder. You need a new set of rings and to have it rehoned. Worst case you'll have to go oversize with a new piston (make sure they match the weight of your other piston assemblies).

If it were me, Id run mineral oil again and run the living daylights out of it. You might be able to save it. Otherwise pull the jug and get it repaired, then follow the break in instructions to a tee. Also get a CHT probe. Its the most important instrument an O-470 can have.
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Skylark60
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by Skylark60 »

This is my first time back here in some time and I would like to thank everyone for their input.

The cylinder was glazed and I now have a spare.

My trip across the country was good in that I used less than a quart of oil in 20 hrs

I did however have a bunch of ground runs thereafter to keep the battery charged, poor decision whereas I glazed the cylinder.

All is good this spring.

My former AME did not follow the service bulletin.
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by RedAndWhiteBaron »

Skylark60 wrote: Thu Apr 15, 2021 12:36 pm My former AME did not follow the service bulletin.
I see what you did there.
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by PilotDAR »

My former AME did not follow the service bulletin.
Just so you know, the standard of work to be accomplished during maintenance is to be defined by the owner/operator of the plane, not the maintainer. Sure, the maintainer is familiar with the nature of the work, and may be hired by the owner to research (AD search, for example) the required background information, but it is the responsibility of the owner to tell the maintainer what work is to be done, and to what standard. The maintainer will sign for the work accomplished.

If, as the owner, you have complied an appropriate maintenance checklist for the plane (required by CAR 625), and the appropriate reference information (AD's Service Bulletins, and ICA's) is identified on that checklist, it makes it really easy for you the owner to describe the work you would like done, and the standard to which it is to be done to the maintainer, and have that person sign for the work as having been accomplished in accordance with the applicable standards.
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by digits_ »

PilotDAR wrote: Thu Apr 15, 2021 2:51 pm
My former AME did not follow the service bulletin.
Just so you know, the standard of work to be accomplished during maintenance is to be defined by the owner/operator of the plane, not the maintainer. Sure, the maintainer is familiar with the nature of the work, and may be hired by the owner to research (AD search, for example) the required background information, but it is the responsibility of the owner to tell the maintainer what work is to be done, and to what standard. The maintainer will sign for the work accomplished.

If, as the owner, you have complied an appropriate maintenance checklist for the plane (required by CAR 625), and the appropriate reference information (AD's Service Bulletins, and ICA's) is identified on that checklist, it makes it really easy for you the owner to describe the work you would like done, and the standard to which it is to be done to the maintainer, and have that person sign for the work as having been accomplished in accordance with the applicable standards.
Is it not the AME who declares the work has been done in accordance with the applicable standards of airworthiness?

Would any AME be willing to sign off if I told him "Hey, can you install this cylinder, but do NOT want you to follow this applicable SB"?

Is "the owner didn't tell me about this applicable AD or SB" a proper defence for an AME who screws something up?
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Re: Continental O-470K, loosing compression on number 4 cylinder

Post by photofly »

digits_ wrote: Thu Apr 15, 2021 3:22 pm
PilotDAR wrote: Thu Apr 15, 2021 2:51 pm
My former AME did not follow the service bulletin.
Just so you know, the standard of work to be accomplished during maintenance is to be defined by the owner/operator of the plane, not the maintainer. Sure, the maintainer is familiar with the nature of the work, and may be hired by the owner to research (AD search, for example) the required background information, but it is the responsibility of the owner to tell the maintainer what work is to be done, and to what standard. The maintainer will sign for the work accomplished.

If, as the owner, you have complied an appropriate maintenance checklist for the plane (required by CAR 625), and the appropriate reference information (AD's Service Bulletins, and ICA's) is identified on that checklist, it makes it really easy for you the owner to describe the work you would like done, and the standard to which it is to be done to the maintainer, and have that person sign for the work as having been accomplished in accordance with the applicable standards.
Is it not the AME who declares the work has been done in accordance with the applicable standards of airworthiness?

Would any AME be willing to sign off if I told him "Hey, can you install this cylinder, but do NOT want you to follow this applicable SB"?
Depends on what the SB says. You choose what work is done.
Is "the owner didn't tell me about this applicable AD or SB" a proper defence for an AME who screws something up?
Non-compliance with an AD or SB is not necessarily “screwing something up”. It’s the owner’s or operators clear responsibility to specify what work is to be done.

If the SB says how something should be done, the AME has to follow it, or equivalent practices:
Persons who perform maintenance or elementary work are required to follow the manufacturer’s recommendations, or equivalent practices.
AD compliance is as much a what as a how: so it would be up to the owner to specify which ADs are required.
Do you know what “the applicable standards of airworthiness” in a release actually refers to?
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