Can an exhaust valve burn if it's not leaking?

Discussion in 'Maintenance Bay' started by Wrench978, Nov 10, 2019.

  1. Wrench978

    Wrench978 Pre-takeoff checklist

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    I am in the process of buying an E-series bonanza. the mechanic doing the pre-buy said that the number 6 exhaust valve is burnt. but he also said the compression was 73/80. the engine just had a top end overhaul last year (about 40 hours ago). My understanding of the work done is that one cylinder was replaced, and the others were removed, inspected, cleaned and reinstalled (and that this constituted a top end overhaul). Im guessing that means they all met certain specs?

    When I brought the cylinder issue to the seller, he was advised that, with the cylinder only having 40 hours on it and the compression being what it is, there is no way the valve is burned. Any thoughts? Im not against needing to overhaul the cylinder, but I need to account for that in the price/budget...
     
  2. MarkH

    MarkH Pre-takeoff checklist

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    Did the prebuy mechanic borescope it and take pictures?
     
  3. Greg Bockelman

    Greg Bockelman Administrator Management Council Member

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    I am not sure that constitutes a top overhaul in and of itself. If all the parts met service specs, I agree it meets the definition.

    As for the burned valve, a bore scope inspection will tell the tale.
     
  4. Checkout_my_Six

    Checkout_my_Six Final Approach

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    post the pictures.....something is not lining up.

    If the valve is really burnt....it won't seal. It may be starting, but then, it isn't yet burnt. Burt means it's warped....like this. Once warped they will not clean up to be reseated.

    .....and if it is "starting" to burn it can be corrected. My guess is a failed rotator....not allowing the valve to spin with each cycle. That creates hot spots and begins the process. You could have it lapped in place (rope trick) and a new rotator installed ($35).
     

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    Last edited: Nov 10, 2019
  5. Wrench978

    Wrench978 Pre-takeoff checklist

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    I’m working on getting more info from the mechanic. I’ll post it here when I do
     
  6. hindsight2020

    hindsight2020 En-Route

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    Where are you gonna hangar that thing down in the PR? Last time I asked, well, there's a reason I live in TX instead of home. Can't afford to own down there. Insurance alone was out of this world. Magnesium V-tail in the island would also not be my first go to, but your monkey your circus of course.

    --
    As to your questions, there's no such thing as a top overhaul. It's a made up term. Holds no weight. They basically inspected the cylinders and re-installed as required. Those that required replacement of components were re-worked as required and re-installed. That's the extent of an "overhaul" of cylinders. Granted, taking perfectly good cylinders and putting new ones in prophylactically, also could be described as a top overhaul. Perhaps the kind of "top overhaul" people often associate with the term and would place a higher premium on. But it doesn't mean anything beyond that, as the components of the bottom end were not overhauled IAW regulation potato, which is the only "overhaul" that counts (SMOH etc etc).

    Burnt exhaust valve is no joking matter, but it's a fact of life with Contis. See Gary's ordeal. Replacing a valve is not a big to-do cost wise in the 30,000ft view of sole ownership, but if that kind of occurrence is going to give you pause, I'd reconsider continental engine ownership.

    We're talking about 1ish AMU between the valve a la carte replacement and the R/R, provided the rest of the cylinder kit is good to go (aka no broken rings, scored walls/piston head, cracked jug etc). A round off error in the context of what that thing is gonna cost ya going forward. Good luck!
     
    Last edited: Nov 10, 2019
  7. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser! PoA Supporter

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    How did the mech decide it was burned (burnt?) ?
    Ask if he heard leakage out the exhaust. (I find that faster, easier, more reliable than the borescope)
    I quantify leakage by placing my cellphone mic in the center of the exhaust stream, a consistent distance from the exhaust pipe and the free dB app will record the sound level...the ex leaks I had were instantly obvious (and the cellphone is way more sensitive than old ears that have listened to thousands of hours of engine operation)
     
  8. Bill Greenwood

    Bill Greenwood Pre-takeoff checklist

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    I would ask the a and p what tells him the valve on 6 is burned, sound or borescope, or oiji board? Its hard to see how it could be much burned and have such good compression. And its the compression that makes the explosion in the cylinder normal power.
    Continentals do often have burned ex valve problems and need to oh or replace cylinders. As for there being no term for top oh, nonsense.
     
  9. hindsight2020

    hindsight2020 En-Route

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    i was just saying it's superfluous. it doesn't have the meaning as smoh time of the bottom end.
     
  10. PaulS

    PaulS Final Approach

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    Yes, a valve can burn and not leak DURING THE COMPRESSION TEST. Sometimes you can have an issue with the valve guide or lifter that is intermittent, causing the valve to stick occasionally, causing burning. Eventually the issue or the damage to the valve will cause it to become chronic and bad things will happen. I'd get it fixed.
     
  11. Bell206

    Bell206 Pattern Altitude

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    FYI: while the term top overhaul is used widely it can't be used in a logbook sign off. Replacing only the cylinders is technically a repair/alteration depending on replacement parts and is signed off as such.
     
  12. Snowmass

    Snowmass Filing Flight Plan

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    At 73/80 it's barely leaking. In fact if you fly and re-test the problem may go away. This is common. In any case if it is seriously leaking it will start to burn so you can wait until it gets into the sixties. Pulling a cylinder and doing a valve job is not that big a deal. I have done it many times.

    As to the original question I do not see how it can burn if it doesn't leak for it is the hot exhaust gasses blowing through the leak in combination the lack of cooling seat contact that causes burning. IOW this would have effect preceding cause.
     
    Last edited: Nov 12, 2019
  13. flyingbrit

    flyingbrit Pre-takeoff checklist

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    Valves usually burn because of guide issues. I suggest you have the mechanic perform a wobble check. If the clearance is excessive, the valve/valve guide likely needs replacing which means pulling the cylinder. If the clearance is too small and/or the valve is sticky a guide reaming may help. This can be done in situ and has Lycoming's approval. If clearance is OK then fly a few more hours and re-check.
     
  14. hindsight2020

    hindsight2020 En-Route

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    Its a conti jug, not lyco.
     
  15. Wrench978

    Wrench978 Pre-takeoff checklist

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    thanks for sharing your knowledge with a rookie! still waiting on borescope pictures from the mechanic, but at this point I am going forward with the purchase. the plane is perfect for us, and while it needs some work and TLC, the seller worked with us to get the price down to within our reach. Ill post pics when it is official!
     
  16. Dr. O

    Dr. O Pattern Altitude

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    Hey, if owning an airplane was affordable everyone would be doing it.
     
  17. swingwing

    swingwing Pre-takeoff checklist

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    So if the exhaust valve is leaking and the cylinder is off the engine what is a fair price to have it lapped or replaced?
     
  18. Checkout_my_Six

    Checkout_my_Six Final Approach

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    3-4 hrs of shop time.....could be more if it's a difficult airframe (re-install).
     
  19. Snowmass

    Snowmass Filing Flight Plan

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    I have personally done this several times. Pulling the cylinder itself is fast so most of the time is in removing the manifolds but 4 hours is reasonable if no problems. However, you will probably need a replacement valve, seat and guide. A good used valve is OK. So add on maybe another $400 for these. Still the cost is trivial by aviation standards.