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Engine to Engine

By Eric Cobb

While doing my usual maintenance every twenty five hours by changing the oil, cleaning the oil filter, and checking in the engine compartment, I noticed something different in the oil  filter.  A metal that I had never seen.  I usually get some carbon and a few very small pieces of a brass looking metal.  This was something looking like aluminum flake - the largest piece being about one eighth inch in diameter, about a sixteenth of a tea spoon worth.  I went ahead and cleaned the filter and replaced the oil.  Told myself I would check the screen after a few more hours of flying.  This is what I did some five hours later and this time I found the screen full of metal flake.  Too much to ignore, so I started taking the engine apart to see what was making the metal.  This has always been my worst nightmare but knowing it is inevitable.  My first thought was that maybe the oil impeller was giving up and making the metal but after removing the accessory case and inspecting the oil impellers they looked brand new.  I continued removing parts and found the bearing on number three rod at the crankshaft had started excessive wear but not enough for what I was seeing in the screen.  After I split the case I found the main bearing showing most of the wear.  I took the case, bearing, rods, crank to Nickson Machine Shop in Santa Maria, CA.
 
  

By the way, I have never heard anything but the highest praise for these guys and I have found that the people in the know about aircraft engines tell me that they are world famous for their work on engines.  I meet Heinz Hubbert, who gave me the complete tour of the operation.  Very interesting.  Heinz told me that the case was fretting.  Which means after hundreds of heats and cool downs the torque on the bolts loosen and the bearing start moving.  This is what had happened and where the metal was coming from.  While I had the engine apart I started inspecting the cylinders and found a big crack in number three piston.  Almost fifty percent around the piston.  It was ready to break at any time. 

   

This would account for the fine pieces of metal I would sometimes find in the screen.  My guess is that it was too fine for the screen to capture completely so it never looked as severe as it was at oil changes.  I have taken the cylinders to Ly-Con in Visalia, CA for inspection and replacement of piston and rings.  I was running 9.6-1 compression pistons.  I love the extra horse power but opting for longevity I will be going back to a standard piston.  This engine was built up by Sacramento City Collage A&P School back in 1984 and I put 2200 hours on it. Because of liability, the school has long stopped building engines for the public.  Hopefully after inspection and rework, it will give me another 2000 hours.  My new motto is, “When the engine speaks, I listen.”  Also I will be doing engine oil analyses at oil changes.  This might have given me advance warning about the piston.  Happy flying and change that oil.  I have never heard of anyone hurting an engine by changing the oil.  Eric Cobb