QUESTION: Last year I bought a 1971 C172L model with a Lycoming O-320-E2D. When I bought the plane I did an annual at the same time and for the most part everything was fine.
This year it was discovered that the camshaft had been damaged along with lobe wear on the No. 4 cylinder. I flew the plane about 75 hours this year, changed oil at 25 hour intervals and preheated in cold weather with a Tanis Heater — both cylinder and oil pan. I have a GEM engine monitor that reads CHT and EGT on all four cylinders and there were no abnormal temperature recordings in either EGT or CHT.
Any ideas what might have caused this type of failure?
South Porcupine, Ontario
ANSWER: Thanks for your question, but it lacks some very important information that would play a vital role in my response. One of the key factors that plays a critical part in the condition of your engine is the total time since the engine was originally put in service, either since new or overhaul. Not having this information available, I’ll assume this is the original engine and that it’s got high time.
While you did the correct thing in having an annual done when you bought the aircraft, what you eventually discovered probably would not have been found during a routine annual inspection. The only way you may have had a possible hint there was a situation with the cam lobes wearing would have been by close inspection of the full flow oil filter element, providing the engine was equipped with one. Most of the original engines bought by Cessna in the early 1970s did not have an oil filter, but rather a pressure oil screen (since I was in OEM sales at Lycoming at the time and Cessna was one of my accounts, I know this to be fact). The installation of an oil filter was available as a conversion, as it is today for engines that only have the screen. Because the wear material is so minute, it is rarely trapped in the pressure screen, but is more likely to show up in the oil filter element if inspected closely. This may also explain why you were able to fly the 75 hours you mentioned, including the 25 hour oil changes, and not have noticed anything out of the ordinary.
You mentioned preheating during your ownership, but not what the past history of that process was. It could be that previous owners left the preheater on all the time during cold weather, which is among the worst things that can be done to an engine. A preheater left on simply becomes a giant condensation generator resulting in internal engine corrosion.
Regarding the readings on your GEM system, I wouldn’t expect you to notice any unusual readings that would indicate a cam-related problem.
No, you did not overlook anything, so don’t blame yourself. It’s just the nature of the beast that the cam lobe wear situation is usually difficult to detect.
Even though you didn’t mention what corrective action has been taken, I assume you did a complete engine teardown and either repair as required or overhauled the engine. Depending on the amount of wear, I hope you made certain the engine oil cooler was also thoroughly cleaned and/or replaced so that any contamination was not circulated through the repaired engine.