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Camshaft Pitting or Debris

in General Tech Discussion 11 years ago
Dear Community

@ all 912UL camshaft experts:

Please have a look at the attached picture. I am just in progress of overhauling my
pistons and cylinderheads. Everything looked good until I had a glimpse through the
bushing of the hydraulic tappets. On 6 of 8 cams I found a light brown discoloration.
First I thought that the surface of the cam is slowly starting to erode (pitting).
Strange to say that all Hyd.lifters are ok and show no signs of unusual wear.

Did anybody of you experience any kind of debris or sediments on the cam? The color
at least looks familiar.

Is the Rotax 912UL cam armed with a thin coating which can start spalling, eventually
excavating a light brown raw material?

Thanks a lot in advance

G.S.


912.Nockenw3.JPG (You do not have access to download this file.)

  • Re: Camshaft Pitting or Debris

    by » 11 years ago

    Gerhard,

    The color is not unusual. Perfectly good cams will develop that light blue and brownish color on the lobes. That small area in the middle of the lobe though, does look as though the hardcoat has worn away. Don't be fooled by the undamaged lifter. It is hardened also and may not have started to wear yet. It will. You may have caught it just in time. Once the hardcoat is compromised, It will very rapidly get much worse. A lobe can go from good to ruined in a matter of minutes if the hardcoat is broken.

    Could be something on the surface of the cam. You might try scraping it with piece of untempered steel or a brass punch and see if it comes off. You might get lucky.

    Bill.
      
    Report

  • Re: Camshaft Pitting or Debris

    by » 11 years ago

    I'm with Bill and see if it is just a discolored spot or actually down into the metal. Whatever you do don't put a scratch in it. Use something soft.

    What type oil are you using and how many hours on the engine?

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Camshaft Pitting or Debris

    by » 11 years ago

    Hi

    The engine 912UL was installed in 03/2000 and served 1350 engine hours in a flight School.
    They did proper maintenance and rarely used 100LL.
    I am the 2.owner and flew another ~ 500h. Since then I never flew shorter than 60min always
    warmed up the engine before t/o. and never exceeded max rpm.

    Oil change was done even before the rec. 100h and I used Motul 10W40.

    Maybe a year ago the engine s max performance/rpm started to drop.

    Compression on cyl. was well below the recommended Rotax value.
    (7,9 - 8,0 - 8,0 - 8,1)

    Therefore I disassembled the cylinderheads and equipped the pistons with new rings
    and replaced 2 valves. Then I saw the cams through the bushings.

    I am sure that the cams are not worn out in a manner that it affects the valve-travel.

    Could there nevertheless be a conection between the discoloration of my cams and
    the compression (engine performance) ?

    G.S.
      
    Report

  • Re: Camshaft Pitting or Debris

    by » 11 years ago

    Hi Gerhard,


    Before you jump in too deep in worry,
    Did you do a leak down differential compression test or only a static? A differential test may be more telling and informative. You can use 80-87 psi for the compression test pressure. Most of what I see in my shop is 87 (test pressure) over 83 to 85. Some gauges aren't overly accurate, but they should be close.
    If you use 80 psi test pressure then it is usually over 77-78 psi. These are fairly normal and there can be slight differences depending on the test gauge. I'm not much of a static test guy. It has its uses, but I rarely use static and I prefer differential compression test.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Camshaft Pitting or Debris

    by » 11 years ago

    Hi Roger

    The leak-test lastly generated 0% loss of air pressure on all 4 cylinders.
    I used a device with a single
    gauge which can be finetuned before operation. Operating pressure was 8 bar (116psi?)
    Strange but true. 30% loss on the static compression test compared to a new engine.
    And 0% loss on the leakdown test.

    Today I got my 4 cylinderheads back.





    I told the boss about the discoloration of my cams and he agreed to accompany me
    to my engine. He looked inside the bushing with a torch and said this is nothing to
    worry about since this does not look like pitting to him. After his conviction these are
    sediments of oil. The reason why you find them exclusivly in the center of the lobe
    is because the surface of the lifter is frequently of concave shape. I have no idea
    if this meets the truth but why should he tell fairy tales? It´s his profession and
    he also stated that one will always find traces on the adjacent lifter.

    quot.: "Zusammenbauen und Spass haben" (assemble and have fun)

    What do you think?
    121120131098.jpg (You do not have access to download this file.)
      
    Report
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