The Cessna 120-140 Association Forum

Hey all,

This one might get a bit long, so summary up front is that I found a concerning (to me and my mechanic) amount of debris in my oil filter when I changed my oil after Oshkosh. I'm at a bit of an impasse on what I should do next and am looking for some opinions and suggestions or maybe just commiseration.

Engine info:
-C85-12F with stroker mod per SE00979AT (O200 crank, rods, and pistons, still placarded 85hp).
-"TAF-L" oil filter modification per SE7559SW (filter installed behind cylinders on left side, removed suction screen).
-Roughly 1300 hours on the engine since overhaul done back in 1997.
-I've owned the airplane for 4 years, fly about 50-75 hours per year, and change the oil every 25hrs running Phillips XC 20W-50.

I did an oil change when I got home from Oshkosh this year and got a heck of a surprise when I cut open the oil filter. Lots of sparkly debris in the filter element as can be seen in the attached pictures. All pleats looked like these. None of the shiny metal particles are magnetic, and from their color, appear to be aluminum. The other dark debris appears to be carbon since it can be crushed with a fingernail. All of my previous oil changes have had spotless filters, so something happened in the last 25 hours. I had no operational indication of problems. Oil pressure has always been and is still right where I'd expect it to be: 50ish psi within 10-15s of cold start, 43ish psi in cruise, and 18-20psi at 800rpm when hot. The engine starts and runs great and performance has not noticeably changed. However, in this 25hr oil change interval, I noticed that oil consumption has increased. This engine used to go through a quart every 25hrs, and is now more like a quart every 10hrs. Not bad overall, but still a large percent increase and it's pretty clearly coming from the breather and not out the exhaust pipes (drips from the breather tube at the end of a flight, oil on the belly behind the breather tube but not behind the exhaust pipes).

I sent the filter element in to ASA labs for a filter patch test (I've been using them for oil analysis). Their filter patch analysis didn't tell me anything I didn't know from looking at it myself. They could do SEM analysis of a single particle from the filter, but it was very expensive and seemed like it'd be really hard to pick a single representative particle, so I declined that option. Incidentally, the oil analysis came back normal despite the debris in the filter. I borescoped all 4 cylinders and saw nothing obviously wrong. Cylinder #4 (front left) had some odd looking staining in the cylinder walls, but nothing that looked like scores/striations. Good oil pressure and non-magnetic debris all seems to indicate that it's probably not bearings/cams/accessory problems. Good starting/idling/power seems to indicate that it's probably not valves/guides.

Talking about all this with my A&P, he and I agreed that I should fly the airplane with fresh oil and filter for 5 hours, cut open the filter again, and see what I find. I did that, and the filter was essentially spotless. Maybe 3-4 individual aluminum specks in the entire filter, but considering the quantity that was there previously, it seemed like that could have just been leftover in the oil passages.

So now I'm stuck with an engine that I really don't trust, but isn't actively misbehaving. I really don't want to do exploratory surgery on it with all the risks that includes. Maybe this is just my indication that the engine is due for overhaul? Curious to hear if other folks have seen this kind of filter contamination before and if so, what was done about it.

I don't suppose you have pictures of the odd looking staining in the cylinder wall of #4, do you?

Hmmmm...............Hmmmm.

I would be concerned too if I saw a filter like that.

Chrome cylinders by any chance? That would be my first guess.

2nd guess would be something "kinda solid" got wound through the oil pump and got deposited everywhere.

I like Blackstone, they do a good job of breaking down the individual metals.

So you've done 5 hours and found almost nothing. Try 10 hours and see what that yields. It's almost as if something Aluminum..?? rubbed on something, wore itself through and called it quits and went back to behaving. (that's really technical!!! )

All of the items I can think of that cause real problems are magnetic. However a piston pin rubbing on an abrasive cylinder wall will do that. At what "O'clock" position is your odd spot in cylinder #4? A good borescope will show many things.

Also, something may have gotten in a cylinder and scored a piston, that will do that. About 10?? years ago, champion had issues with their spark plug porcelins coming apart and bouncing around inside the combustion chamber. I saw oil filters that looked like that back in "those days".

Just "spit balling here......."

I would pull the starter and then fish the borescope into rear part of the crankcase thru the various holes. That is how I found a cracked center crank journal when I was getting metal in my oil screen. You can also see some of the cam lobes. I don't think you can fish the borescope into the front of the crankcase with this method but at least it's something.

Is the discoloration on cylinder 4 in the same area as the piston pin?
Josh

My borescope is kind of terrible, but these are the pictures I got of cylinder #4. The borescope is coming in through the top spark plug hole and oriented approximately level with the crankshaft axis. Piston is at bottom dead center. The larger sort of arrow shaped stain in cylinder #4 is slightly above 8 o'clock position and the two smaller stains are approximately 7 o'clock as you'd stand on the left side of the airplane looking at the top of the cylinder. Wrist pin caps was the first thing my A&P thought of when I told him I had aluminum in the filter, but if these marks were caused by a dragging wrist pin, I'd expect them to be right on 9 o'clock and extend much farther down into the bore past the top of the piston. As a comparison, I included a picture of the adjacent cylinder #2 which appears flawless to me.
To the best of my knowledge they're not chrome cylinders. I only looked at the top 4 plugs since I had them out and they all appeared consistent color with no obvious missing chunks from the ceramic. Mag checks are fine too, so I would expect similar from the bottom plugs. That said, I did have the generator replaced with an alternator last annual which was within this 25hr window. While I'd hope not, I suppose it's possible for something to have been dropped into the accessory case during that work?

Bill, is the crosshatching gone in cylinder 4? I can clearly see the crosshatching in cylinder 2.
Josh

A chrome cylinder should have a painted band around the base of the cylinder, orange, IIRC.

That cylinder does look like it's worn through the cross hatching. I'd be thinking about pulling it for inspection unless I found some other smoking gun.

The suggestion to pull the starter is a great one, also pulling a magneto is easy and painless to look around the back of the accessory case.

I've borescoped countless cylinders and never seen a mark like that! John's suggestion to pull the cylinder is spot on.

Thank you all for the responses!

John, while the bases of the cylinders aren't painted, I checked the cylinder serviceable tags in the logs and all of them had the CermiNil process applied when the engine was overhauled in '97. Cylinder #2 was pulled in '01 with just a note that the bore was honed (along with a bunch of valve guide and seat work). Probably why the crosshatching is more visible on that one than the others. I agree that the cross hatching appears worn through on #4 and I wonder if the stains are maybe spots where the CermiNil coating has flaked off or worn through? For what it's worth, compression on #4 last annual was 76/80.

I think my plan of attack for now is to fly the airplane through the fall here which should net me ~10 hours on the oil and filter. My annual is due in December, so at that time I'll check the filter again for contaminants and talk with my A&P about pulling #4. And if we don't pull that cylinder, at least go in through the starter with a good borescope. There's a small oil leak there anyway, so could address that at the same time.

Cermi-nil is a completely different process from Chrome plating. It is plausible that the bits in the filter are from the markings on the cylinder wall.

Watch for a spike in oil consumption.