A brain teeser for the analytical types on engines

[cliffy]

This is what my engine was saying a few years ago. Anyone want to venture what the problem was?

I started to notice breather oil on my belly when I had never had any before. Then, on one flight I noticed a loss of 7 pounds of oil pressure all of a sudden. Still above mins all the time. At the same time I noticed my left mag just slightly rougher inflight than the right. Enough so only I could tell. It was oil change time so upon draining the oil I noticed fine aluminum powder in the oil. Sent out an oil analysis but it came back as normal. Sent in the filter for microscopic check and it came back normal.

What would you do?

What was wrong?

[Piloto]

As your brain got older you put cooking oil in it by mistake.

José

[cliffy]

Just going through my old posts and surprised that no one wanted to chime in on a real problem. It did happen and was an eye  opener. I grounded the airplane. Cogitated about the problem and torn the engine down (O-360). When I removed the rear case the left Idler gear and support post fell off in my hand. This "post is tied into the oil galley so the gear axel has lube. The 2 1/4X20 bolt that hold it to the crankcase had worked loose and wallowed out their respective holes to 2X their normal size. This is where the fine aluminum in the oil came from. Because this post opened up the oil galley the oil pressure went down as the oil sprayed into the rear case area of the engine. because the breather comes off the rear case all that oil spray started to come out the breather where I had no oil before. Because the LEFT idler gear was loose the left mag changed timing slightly all the time causing the slight roughness. Moral of the story? If the engne is talking to you-LISTEN and do your own analytics.

[KSMooniac]

Very interesting!  I missed the post the first time around... you are correct that we are our own best diagnostic tools once we get familiar with the operation of our planes and know the sounds, vibrations, oil use, etc.  Listen when it talks!

[Sector95]

Did the bolts augering holes ruin your block?  Or were you able to fix it?

[cliffy]

I was able to send the crankcase to Tulsa and have it welded and redrilled. There is a service letter that shows to use the next size bolt also, which I did.  1/4-20 up to 5/16-18 on both of the idler gear studs.

[DaV8or]

This is an excellent testiment as to why an oil annalysis and cutting open a filter is not the final word to determining engine health. On a recent thread on the AOPA forum about running past TBO, the concensus was that as long as the oil annalysis came back clean and the filter turns up clean, there is no reason to ever rebuild an engine and the 2000 hours that the FAA and Lycoming recommend is hogwash. I personally think the TBO number is kind of like an annual. It's not about rebuilding because it's all worn out as much as it is getting in there and doing an inspection to all the things like idler gears coming loose, or perhaps cracks in the case that you can't see on the outside.

On an engineering note, I'm surprised that the bolts holding the idlers on aren't fine threads. I think their use of a course thread bolt contributed to the loosening, but I guess we are talking 1950's tech, so...

[cliffy]

I agree completely on the thread pitch issue. The "strip out" factor with the base metal being cast aluminum may be a factor in the thread pitch. Also I would have thought that someone would have thought to use drilled head bolts and safety wire them in place so that they had less chance of coming loose. Also from the start, as a practicle engineering matter, I would have used the largest bolts for for available space right from the begining.

Oil samples and their analysis may not always be the final or correct answer. To rely on one "answer" completely is nothing more than burying your head in the sand. Had I "listened" to the oil sample alone, I would have lost the engine and probably the airplane. I have preached for years (been doing this since the late 60s) that engines and airplanes talk to you. You just have to learn the lingo and pay attention.