When is it time to get an overhaul?

[rainman]

My plane is in for it's annual. I was just informed that my 1979 231 with about 875 hrs on a factory new engine now has two cylinders with low compression in the 30s. The engine was replace in 2000ish. No problems at the last annual, now this. Do I just do the two cylinders? Do I do all six and a top? Should I just bite the bullet and do a major. I plan on keeping the plane for several more years. What questions should I ask to help decide how to proceed? Ray

[N201MKTurbo]

I see no reason to do a complete overhaul. If it was my engine I would have the cylinders repaired individually.

 

I would pull the bad cylinders and determine the cause of the low compression. Then decide weather you want to overhaul them all. 

 

There is a certain overhead with pulling cylinders that make it more economical to do them all at once just so you don't have to do it all over again next year for additional cylinders. It is a tough decision.

[chrisk]

This is consistent with what I have generally heard about the 231.  Basically that they need new cylinders around mid TBO.  For me, the question would likely come down to the cost to do all 6 cylinders now, versus 2 now and 4 later.  --So, I would look at a top, not a major.  The question being 2 vs 6.   Then I would look at new vs overhauling the existing cylinders.  

 

Also, your not very far from me.  I'd really like to know who you get to do the work, what it costs, and your experience.  At this point, I'm assuming your plane is considered un-airworthy (some what depending on paperwork).  Presumably you can get a ferry permit to take it to the shop of your choice.   I know I'll eventually go through this same thing.     

[Bob - S50]

Not an A&P, but if it were me...

 

Does the plane fly OK?  If yes...

 

Do a borescope and look for the reason for low compressions.  If you don't find any, fly it and check the compressions with the engine warm.

 

Then decide.

 

Bob

[N201MKTurbo]

 Do I do all six and a top?

 

Doing all six is a top.

[carusoam]

There is a similar discussion in recent history here...

Search for Erik Bollt's thought process and time line.

It would be bad form to throw the baby out with the bath water.

Run and retest the compressions. Make sure what you see is real.

Expect that cylinder maintenance is the minimum, a top OH would not be too far out of line if you wanted it.

Check out the JPI history on the two offending cylinders to make sure they don't have a history of being hotter than the other four.

Then decide how many to replace...

Then decide OH the cylinders or buy new....

So many questions...

Best regards,

-a-

[1964-M20E]

Not an A&P, but if it were me...

 

Does the plane fly OK?  If yes...

 

Do a borescope and look for the reason for low compressions.  If you don't find any, fly it and check the compressions with the engine warm.

 

Then decide.

 

Bob

+1

[aaronk25]

Have a qualified person look in the cylinders with a borescope camera. If no smoking gun, run it. How is oil consumption? Any metal in oil filter? If all ok keep running for check in a few hours. Continental cylinders do this often.

I hate new parts you never know if the part your getting will fail. I'd opt for repairing the cylinders, if required.

Continental has a service bulletin out about this.

Google Mike Busch all about cylinders.

[Skywarrior]

Compressions can easily vary between checks, just due to the varying positions of the rings in each piston between checks.

 

But... TWO cylinders showing low compression at the same time is an outlier, so to speak. Those cylinders do need to be borescoped. To my Holiday Inn Express mind, the problem could well be eroded exhaust valves.

 

This situation is a good rationale for spending the money on an engine monitor with downloadable data. I'm looking at getting an MVP-50P myself.

[Seth]

As all have said.  Not a major.  Most likely two repairs or replacement of cylinders with an eye watching the other four.

 

-Seth

[FloridaMan]

I would do the two cylinders. They say when cylinders start to go, others follow. Granted, I have the bulletproof IO360, but mine had the first cylinder go around 500 hours ago and the next one this past annual. Out of curiosity, which cylinders are they? 

[aaronk25]

Cylinders die of heat, expression the cylinder head components such as exhaust valves, valve guides and the cylinder head cracking do to larger heat cycles. How hot are you running as far as chts?

[N601RX]

Ask the mechanic to determine if the low compression is due to leaky exhaust valve or around the rings.  Follow this service bulletin, It may be as simple as staking the valves.

 

http://www.swaircraftappraisals.com/MeyersForum/Maintenance/Continental%20SBs/M8415.pdf

[KSMooniac]

Cylinders die of heat, expression the cylinder head components such as exhaust valves, valve guides and the cylinder head cracking do to larger heat cycles. How hot are you running as far as chts?

 

Valves also fail due to poor installation...and Continental is infamous for this, especially in the ~2000 time frame if new factory cylinders were used in the OH.  If that is the case, you should borescope ALL of the valves to look for the heat/deposit signatures.  The other 4 might not be far behind, in which case fixing all 6 valves at once would be more economical.  This doesn't necessarily mean replacing all 6 cylinders, though!  Wait to make that decision until after one or more are removed for inspection.  You might get by with new exhaust valves, and then continue the rest of the TBO fun.

 

*disclaimer, my opinion is contingent on the low compression culprit being leaky exhaust valves!

[Cruiser]

Yes, you need data on the root cause of the failure. Your OEM cylinders only have 900 hrs TT. Getting rebuilt or overhauled cylinders is somewhat of a concern since you don't know how many overhauls and hours are on the replacements. You do know the hours on your cylinders. you might want to investigate having them rebuilt and returned.

Definitely get the rest of the cylinders checked. If they show signs of exhaust valve failure you are just prolonging the inevitable and it will cost you the labor hours to remove the intake and exhaust each time you replace a cylinder vs doing them all at once

[rainman]

Thanks guys, I've got some work to do and need more info before I decide. My plane is at Dugosh and they are planning a boroscope. They are good about communication, and are close to me so I can go over and check things out. I do have a JPI 830 and monitor CHTs closely.

[DonMuncy]

Go to Saavy Aviation's web site and watch/listen to Mike Busch's webinars. He (and I) believe that compressions are virtually a waste of time.

Continental agrees, as they have an SB that says if you have low compression on a cylinder, you should fly it for 45 minutes or so and recheck. Only if it is low again, do a borescope and see if anything is wrong inside.

My 231 had about 4 cylinder changes due to "low compression" before I bought it, and three more after I bought it, before I learned. The last two times well-meaning A&Ps recommended cylinder replacements, I followed Continental's SB, and both times, the cylinders then "passed".

My belief is that the theory "all 231's need a top overhaul at 1000 or so", is in large part because of this misconception about compressions being used for determining when cylinders need replacement.

Unless you just want to spend $1200 per cylinder for fun.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

s

[John Pleisse]

Comps in the 30's on two jugs are smoke suggesting fire and should be below even the threshold of Mike Bush's never ending blessing of wisdom. Make sure the cam is OK and top it. Go fly another 800 hrs. Mike Bush.........whatever.....

 

Might also be a good mid-time pit stop for the turbo and associated parts.

[Bob]

What cylinders are low?

[Guest]

TCM sb 03-3 now superceds sb 84-15. The new sb explains the process clearly.

http://www.tcmlink.com/pdf2/sb03-3.pdf

Clarence

[aviatoreb]

There is a similar discussion in recent history here...

Search for Erik Boltt's thought process and time line.

 

 

Yup - Anthony is right, almost.  (....only wrong part is he spelled my name wrong!  Erik Bollt).

 

I don't know specifically about the TSIO360 in the original 231, but in my 231-rocket, the space is so tight, and there is so much stuff running across the top of the engine - exhaust, baffling, etc, that to pull a single cylinder takes a good bit of shop hours to remove one cylinder.  So when I had one cylinder go soft, I decided to go ahead and redo all the cylinders at once.

 

Also part of the thought process is that the fact that one cylinder went bad was a reflection of poor workmanship from the Continental factory on seating the valves, as was their bad reputation for cylinders/valves from production of that era.  So my presumption was that if one went, then otherwise would go before another thousand hours (I was then at 985 hrs).  Further, I had the brand new cylinders worked by a top shop - Victor aviation - to ensure a beautiful seal to prevent this failure mode.  I will do that in all engine builds I have moving forward. I consider such treatment better than factory new.

 

What I saw when we pulled all 6 cylinders, even the ones that were still showing good compressions, is indeed the workmanship was poor in how the valves seat and the cylinders I was putting in with custom machining in the valves were dramatically better.

 

That was 300 hours ago...and still keep on truckin'.

[aviatoreb]

Go to Saavy Aviation's web site and watch/listen to Mike Busch's webinars. He (and I) believe that compressions are virtually a waste of time.

Continental agrees, as they have an SB that says if you have low compression on a cylinder, you should fly it for 45 minutes or so and recheck. Only if it is low again, do a borescope and see if anything is wrong inside.

My 231 had about 4 cylinder changes due to "low compression" before I bought it, and three more after I bought it, before I learned. The last two times well-meaning A&Ps recommended cylinder replacements, I followed Continental's SB, and both times, the cylinders then "passed".

My belief is that the theory "all 231's need a top overhaul at 1000 or so", is in large part because of this misconception about compressions being used for determining when cylinders need replacement.

Unless you just want to spend $1200 per cylinder for fun.

 

 

 

s

 

I think you are right that some rebuilds are due to a flaky compression test that would bounce back if flown for 45 min.   Maybe if a cylinder is showing upper 50s or 60s I would believe that.

 

In my case two years ago, I had my compressions go from all 6 in the mid 70s to one in the 20s since the previous test.  And there were also signs of valve heat damage due to leaking gases.  I was very happy to have that cylinder off once I saw its insides.

 

By the way, for anyone interested, it is possible to pull a cylinder and send just that cylinder in for an overhaul for something like $500.

[Mike A]

I think you are right that some rebuilds are due to a flaky compression test that would bounce back if flown for 45 min.   Maybe if a cylinder is showing upper 50s or 60s I would believe that.

 

In my case two years ago, I had my compressions go from all 6 in the mid 70s to one in the 20s since the previous test.  And there were also signs of valve heat damage due to leaking gases.  I was very happy to have that cylinder off once I saw its insides.

 

By the way, for anyone interested, it is possible to pull a cylinder and send just that cylinder in for an overhaul for something like $500.

 

I just had the #1 cylinder on my -LB removed, overhauled, and reinstalled for $1200 all in.  The overhaul itself was $875.

[helitim]

Erik,

Where can you get a cylinder O/H'd for $500? My top 2 months ago cost almost 1K per cylinder. That did include new valves/guides/pistons/rings, etc...

Tim

[aviatoreb]

Erik,

Where can you get a cylinder O/H'd for $500? My top 2 months ago cost almost 1K per cylinder. That did include new valves/guides/pistons/rings, etc...

Tim

 

Hi Tim,

 

That was just a number off the top of my head from memory from shopping for a cylinder overhaul 3 years ago.  I see above someone said its $850.  So that's roughly in line with my $500....vs $10k for the top I spent $500 or $850 all sounds like less than $1k and that is all I remembered 3 years later.