Low Compression on 2 Cylinders

[KenyaSkies]

I own and fly a Cessna 170 ragwing with 850hrs on the engine. Two cylinders were below 60/80 on the last 100 check. I am looking for suggestions please on the best way forward: a) Two new cylinders b) Two used cylinders d) Six new cylinders.

[juredd1]

I'm no expert so I probably should keep my mouth shut but why do you need any new or used cylinders? Why can't those two cylinders just be reworked or overhauled?
Has the plane been flown regularly of late or has it set for any length of time? Just asking because when I let my 152 set for very long I had compression issues.

Justin

[johneeb]

Nick,
What was the "Master Orifice" reading for your compression tester when you checked your cylinders?

[GAHorn]

Hi, Nick!
Low compression does not automatically mean you've got to do that kind of drastic work, you know.

It's important to know WHY the compression is low... For example, ... did you hear air escaping from the air filter? (intake valve leaking)

Or from the exhaust tailpipes? (exhaust valve leaking)

Or from the crankcase oil filler cap? (piston rings leaking)

In comparative order: The first could be carbon under an intake valve that needs cleaning/staking.
The second might be the same..or more likely a burned exhaust valve (and is the most serious concern, IMO)
And the last could be a STUCK ring... or simply ring-gaps temporarily aligned...that very last requiring no action at all for a while, and while the stuck ring might be easily repaired with no parts at all other than a gasket or O-ring.

So... what symptoms exist OTHER than compression a bit low. (just because it's below 60/80 doesn't condemn a cylinder.)

[Metal Master]

Here is a link to the whole story from Mike Busch. And while I do not always agree with Mike Busch he is right on this one. Be sure to read the whole article or you will get the wrong impression of your compression.
http://www.avweb.com/news/savvyaviator/188758-1.html.
The bottom line is that as long as you have no issues with the Intake and particularly the exhaust valve and you have calibrated your Compression tester with the master orifice tool you could have a cylinder significantly lower that 60/80 and not need to remove or repair it. Refer to the service bulletins in Mikes article. TCM Service Bulletin SB03-3.
Jim

[KenyaSkies]

Thanks to all for your very useful comments and advice. I live in Kenya where my 170 is the only one here. The maintainance options are limited and when it comes to pulling the cylinders any number of chain reaction disasters can begin. The compressions that came back from the last 100 service were 5/80 and 32/80 but even that has not been fully confirmed by the maintainance guys who only gave me those readings after serious hounding and finally looking through their notes. I have continued to fly since then and although I am operating mostly below 2000ft there definitely is a lack of power on takeoff, nothing hazardous but enough to be cautious about. It also climbed happily to 9500! The maintainance shop wants to carry out another compression test when the engine is hot... That indicates that they are not sure either!

The plane flies about 100 hrs a year and is running on good old Kenya Super from the gas station - filtered carefully! She I imported the plane from the US 2 years ago - compressions were good on the former 100 hr check.

I will look into all the very helpful suggestions above and continue to monitor further comments and get back to you all down the road. Thanks again for all the useful help.

Nick

[c170b53]

Nick, they should be doing the check post engine run, in other words on a hot engine. As pointed out, if leakage is present determine where the leak is occurring. That info is more important than the numbers.

[daedaluscan]

Living where you do I might be inclined to keep a spare cylinder on hand. I do.

[Bruce Fenstermacher]

Nick, I hope that is a type not really 5/80 .

The 32/80 is a little concerning as well.

As has been documented at the forum lots of things can be done to improve compression without removing the cylinder depending on what the reason is for the low compression. Many leaks are carbon on the exhaust valve and set. That can be cleaned up without removing the cylinder however, you must have the help of an old time mech who knows more about engines than the maintenance manual describes. Cleaning and lapping valves through the spark plug holes has been described here elsewhere.

[ghostflyer]

I would be buying a brand new cylinder Assy. Only as a spare . Superior make up a good kit . I also fly in that type of environment and obtaining spares is quite a pain some times . Everything said before has to be done before pulling a cylinder . But never buy a second hand cylinder even though it's been overhauled . You do not know the hours it's done and what cylinder temperatures it's been to . Plus the cost of overhaul is near the the cost of a new Assy.

[rydfly]

But never buy a second hand cylinder even though it's been overhauled . You do not know the hours it's done and what cylinder temperatures it's been to . Plus the cost of overhaul is near the the cost of a new Assy.

+1 on what ghostflyer said... having been bitten twice now by using overhauled exchange cylinders, I'll never use one again if there are any other options available. As long as there are new cylinders available, that would be my first choice unless I know the complete history of the one(s) being overhauled. Cylinder exchanges are a total crap-shoot.

[KenyaSkies]

Thanks for all the feedback. When I turn the prop both exhausts give a big 'sigh'. They don't do that on the 180 so just wandering if that is a sign?

[bagarre]

Means air is leaking past one or more of your exhaust valves.
Could be a pice of carbon, could be a burnt valve. It's best to put a bore scope down the spark plug hole and have a look at the valve and seat.

[KenyaSkies]

Thanks David.

[KenyaSkies]

To All - I had a second compression test and the numbers are not good. 52; 32; 0. I have delivered the plane to the maintainance shop in Nairobi and they are going to pull the cylinders and take a look. It's looking like I may go with a full set of new cylinders as there are 900 hrs remaining on the engine. He will take a look first and let me know what he finds so I will send an update when I have that. In the meantime here's the question for today: where is the place in the US (or UK if possible) to purchase new cylinders? I communicated with Continental a while back and they quoted $900. Any advice appreciated. They would be shipped to the UK then to Kenya. Thanks - Nick