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Lycoming exhaust valve/guide wear

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  • Lycoming exhaust valve/guide wear

    From the department of “for whatever it’s worth,” I have a heads-up for folks running a Lycoming angle valve engine with exhaust valves/guides with 1000 or more hours of operation. The following may also be relevant to other engine models.

    The engine for my 4-place plans built came from a Mooney M20J (200 hp angle valve 4 cylinder) with 1200 hrs on it since factory remanufacture (zero time logbook). I have put an additional 360 hours on it. Not long ago I got a faulty reading on a borescope inspection and thought I had a burned exhaust valve. I pulled the cylinder and handed it over to a repair shop for IRAN. Good news was that the valve was not burned. Bad news was that both the valve stem and guide were worn way past allowable limits. Valve and guide were replaced and cylinder reinstalled; all good...except. I began to research Lycoming exhaust valve/guide wear and discovered that Lycoming recommends a valve wobble test every 1000 hours for my engine model, which was built in 2000 with newly designed hardened exhaust valve guides. There is a service bulletin and a service instruction on how to do the test; however, it seems that doing it properly requires a fixture that costs about $1500. I could find no evidence in the engine’s logbook that any such test had ever been done prior to my purchase.

    Anyway, with all that in mind, I’m not too surprised that with over 1500 hours since new, the valve and guide were worn. It also seem logical that if the valve & guide in one cylinder were badly worn, then the others were probably in the same condition. So, I pulled the other three cylinders and delivered them to the same shop for IRAN. Sure enough, all three needed exhaust valves and guides and had to be replaced.

  • #2
    I like that you shared this, though I don't like that you had to go through it! What would have been the indication of their wear if you hadn't caught it the way you did?


    • Ray Strickland
      Ray Strickland commented
      Editing a comment
      Not sure about that, but I recall reading articles saying that eventually the valve will not close properly and burn. Not good.

  • #3
    The wobble test was also required on the O-540J parallel valve engine in my Maule at 1000 hours. I watched my mechanic do it, don't recall him using a special fixture.


    • #4
      Great post Ray! Thanks for sharing.

      Lycomings have an outstanding reputation for running for a very long time. If you talk to engine shops they will tell you that Lycomings also show more valve guide wear than other engines. It's not necessarily a flaw but just the nature of the material and engineering the engine was built with.

      I can't remember the Lycoming document number for the wobble test but it is not hard to find. There is also a bunch of information found via google that show the basic wobble test done on other engines. It is pretty simple in concept and an experienced mechanic can judge by feel, no jig required. However, wanting to follow the actual procedure and going by numbers rather than feel isn't a bad thing.

      While the wobble test is more of an approximation of guild condition it does work pretty well. I had a bit of cylinder issues with my Continental. When I was reassembling the valve train I wobbled the valves and did not like what I felt. Since the cylinders were off I measured the guides and were right on the edge of service limits with a couple being out of limits. It was a good time to send out the cylinders.

      If you have a data recording digital engine monitor frequently you can see the loose valve indicated via EGTs. On a cold start, the EGT on the cylinder with the loose valve guide will come up slower. Sometimes the engine will run rough until it warms up. Some call this morning sickness.
      Scratch Built 4-place Bearhawk. Continental IO-360, 88” C203 McCauley prop.


      • #5
        Does the wobble test require pulling cylinders, or can this be done just by removing the head covers?


        • #6
          Wobble test is done with cylinder in place.


          • #7
            Doesn't require pulling cylinders and can be done without expensive tools. I studied up on it and did my daughters airplane 11 years ago, but haven't done any since and don't remember much for details now.