Question for the Mechanics

[W.J.Langholz]

O-300 engine. Compression check and you find 2 out of the 6 with 65-68. Oil useage 1 qt per 4-5 hours.
Would you buy rebuilt cylinders @ say around $400 with exchange or new ones for $1000 with new rings and pistons?
Aprox 3500hrs TT and 1650 since major overhaul. Flying 400-500 hrs a year.
Where would you shop for either new or rebuild?

Thanks
Willie

[Showboatsix]

What were the last two readings on those cylinders? I would be more inclinded to put those cylinders on a 10-20-30 hour test cycle to see what they do before I would tear down an engine. What has the "trend" of all the cylinders been doing?

[Brad Brady]

What were the last two readings on those cylinders? I would be more inclinded to put those cylinders on a 10-20-30 hour test cycle to see what they do before I would tear down an engine. What has the "trend" of all the cylinders been doing?

I agree with showboat.....just because they are low doesn't necessarily mean that there junk......could be a piece of crap clogging a valve....just a bad time for a compression check (all rings lined up to let pressure out) (that one is really out there but is an option) It needs more scrutiny....Any thing over 60 is usable....but needs investigation.....

[ronjenx]

You weren't asking for troubleshooting, but don't jump the gun 65-68.
Where was the leakage? Exhaust, intake, breather?
My brother's 170 had two such cylinders. It was corrosion on the valve faces/seats. (edit to add: the exhaust guides were gummed up too. Reaming took care of that) I pulled both cylinders and took them to the local shop to have the valves/seats touched up. Reinstalled and all were back up to 78/80. Total cost: I don't remember, it was so little.

[W.J.Langholz]

Several other things I notice, oil likes to drip out of the breather tube and the right side exhaust doesn't burn as clean either. I change oil every 25 hours but I have noticed it will gets dark fairly soon pointing to rings or valves????

[canav8]

Willie, A compression check is in order with the engine warmed up. As someone posted. You need to listen to the Oil Filler Neck without the Dipstick, You need to listen to the intake and you need to listen to the exhaust stack. There are only three places where the air can go.

If it goes by the rings you will hear the air pressure hissing in the Oil Filler neck.

If it goes by the intake valve you will hear the air pressure hissing from the air intake or carb. It is loud.

If it goes by the exhaust valve you will hear the air pressure hissing from the exhaust stack. It will be a deep throaty sound.

Your engine is reaching the TBO anyway. The big problem is you can do further internal damage to your crank by running the engine in this condition. If your other cyclinder compression ratios are 25% higher.(ie. 75/80)
60/80 psi should be the lowest compression you should use if you demand a lot out of your aircraft. I will not fly over the Sierra Nevada Mountains when compression is that low. The compression should always be considered when measuring the health of your engine, and what kind of mission you are doing. I hope this helps. Doug

[johneeb]

Willie,
For a little light reading with your Saturday morning Coffee.

DIFFERENTIAL PRESSURE TEST AND BORESCOPE INSPECTION
http://www.tcmlink.com/pdf2/SB03-3.pdf

[mrpibb]

Wille,
Continental give you a generous allowance on the compression test, provided you can determine the area of leakage. As you see mentioned in the previous post there are several areas of exit.
the greatest allowance I believe is past the rings, you would have to check the Continental service bulletin for this. You mentioned a drippy oil breather, if one or more of your cylinders are allowing
excessive combustion gasses passed the rings you may be pumping the oil out thru blow by out the breather tube. A for instance was my O-300 would use about a quart in six, all my compressions always showed high 60s and low seventy's, there were three ( low cyl ) and one in particular would blow passed the rings. I changed one of the cylinders last annual due to in addition to low comp test, it started to show signs of
oil fouling on the lower plug. upon diassy I found the oil control ring and two of the compression rings coked up and seized. so I recondition a spare cylinder threw, a new piston with rings and installed, now even though the other two cylinders are on the low side there not pumping oil as much, I'm now using a quart in every 15 hrs or so and a cleaner underside. Also how much time is on those cylinders? even though you
have a TBO of 1800, cylinders usually start needing attention at mid life.
Just a reminder, are you using the calibrated orifice to set you tester up?

[W.J.Langholz]

Wow, thanks for all the good stuff here in response to my questions!!!!

Always alot to learn...always wanting to learn more

Thanks

Willie

[GAHorn]

All good advice. I'd keep running it, unless I were ready to do the complete overhaul. Even then you might consider having YOUR OWN cylinders rebuilt. (I'd certainly prefer rebuilding my own over an exchange with rebuilts of unknown origin.)

I don't know at what level you presently run your oil.... I used to run mine at 7 qts. It would use a quart every 6 - 8 hours. I ran out of spare oil on a trip and let it get down to 6 quarts. It stayed there the next 18 hours. Guess where I now run it?

[Brad Brady]

All good advice. I'd keep running it, unless I were ready to do the complete overhaul. Even then you might consider having YOUR OWN cylinders rebuilt. (I'd certainly prefer rebuilding my own over an exchange with rebuilts of unknown origin.)

I don't know at what level you presently run your oil.... I used to run mine at 7 qts. It would use a quart every 6 - 8 hours. I ran out of spare oil on a trip and let it get down to 6 quarts. It stayed there the next 18 hours. Guess where I now run it?

There is a minimum take off number for oil (quite honestly for a c-145 or o-300 I don't know what it is) for an o-235 it's 2 qts..........just remember the oil is used for cooling......Like you George I let mine get to six quarts or a little under, then I add a quart.....unless I'm going on a long trip in the summer.

[GAHorn]

Minimum for a C-145/O-300 is 4 quarts. (Min for a Lycoming is 2 qts.)

[HA]

hey Willie, ignore that digression about minimum oil quantity - you're not going to keep it that low anyway. I run mine at 6, add if it's lower, anything above that and it's going out the breather anyway.

compressions - 65-68 is nothing to worry about, UNLESS it's going through a valve. then get a look inside the cylinder (does your mechanic have a borescope? that's best, else I've used a piece of a busted inspection mirror on a flexible parts grabber through one plug hole and a flashlite through the other to see inside the cyl), along with looking at the plugs - no pieces missing? no scrapes down one side of the cylinder wall? no chips or hammering on the plug from loose parts? great, maybe you can just face the valve and seat in place - do a search for the "rope trick" on this forum which is just holding the valve in place with a rope between the valve and piston top so you can remove the valve keepers and springs, then get a LITTLE valve grinding compound on the valve face and reface it to the seat. it's explained better somewhere else here.

if that fixes your air leak then you're good to go after reassembling things. the big thing to watch on compression tests is that unless there is a very obvious problem, you're more interested in the TREND which is why your mechanic is writing the compressions down at each test (I hope). you hardly ever have to take things apart based on a single test. it's only one indication, like oil on the plugs, high oil usage, high CHT, whatever.

[Brad Brady]

hey Willie, ignore that digression about minimum oil quantity - you're not going to keep it that low anyway. I run mine at 6, add if it's lower, anything above that and it's going out the breather anyway.

.

Willie, I guess I should have added, I would rather clean the belly of the airplane than do what I'm doing to Dad's engine right now.....haven't goten to the crank yet......but can read serial numbers on some of the parts in the pan It's just when your uncomfortable with what you see coming down the line (and we saw nothing comming).....Brad

[n2582d]

… You mentioned a drippy oil breather, if one or more of your cylinders are allowing
excessive combustion gasses passed the rings you may be pumping the oil out thru blow by out the breather tube. A for instance was my O-300 would use about a quart in six, all my compressions always showed high 60s and low seventy's, there were three ( low cyl ) and one in particular would blow passed the rings. I changed one of the cylinders last annual due to in addition to low comp test, it started to show signs of
oil fouling on the lower plug. upon diassy I found the oil control ring and two of the compression rings coked up and seized. so I recondition a spare cylinder threw, a new piston with rings and installed, now even though the other two cylinders are on the low side there not pumping oil as much, I'm now using a quart in every 15 hrs or so and a cleaner underside. …

Anybody have experience doing this oil control ring solvent flush? If so, what were the results?