The International Cessna^® 170 Association

Oops! How did THAT get in there? Admittedly, I usually approach cleaning and inspecting the sump screen as more or less a perfunctory chore, but I do it anyway. After all, what are the odds that anything is realistically going to get stuck in there in the first place? I know, I know, we’ve all heard the stories about broken piston chunks and whatnot, but still…
Even doing this for a living back in the 70s, literally doing hundreds of oil changes on dozens of airplanes, I never once found anything that didn’t belong there. Imagine my surprise when, just like always, I pulled the sump screen to have a look and out pops this little broken cotter pin. Yikes! So, off come the mags, also the starter and the generator and…nothing. Everything intact, no broken anything.
History: engine is original to the airplane, about 2,500 total with a questionable rebuild sometime in the 60s and a more recent overhaul by (I hope) a reputable shop 230 hours ago. It runs strong, good oil analysis and is still so clean inside you could lick it. A real dilemma. Any suggestions before I have to start yanking cylinders?

I’ll add this: anyone not doing it routinely, PLEASE INSPECT YOUR SUMP SCREEN!

My guess is that’s it’s a link off the chain to the oil cap. That might be the luckiest and most benign explanation.

You might want to verify whether it is the same size as is used for the rod bearing cap nuts.

I like the oil cap idea.

I also wouldn't be too quick to pull cylinders. Been a while since I did rod end caps but don't seem to remember the leg would or could be as long as the one you found broken off.

Looks to me like a generator drive cotter.

Maybe pull a single cylinder use a borescope to inspect the rods. Might be tricky but it could work. Also, it looks like your missing a part of the broken cotter key.

Is it stainless? Is it magnetic? Does the loop have a “cut” at its’ short leg? (or is it smoothly formed?).
If it’s cut, it might be a discarded cotter that escaped into the sump. (thinking of the conn-rod-nuts)

A boroscope thru the oil filler neck, the sump-drains or thru the suction-screen port might tell if all the conn-rods are properly safetied.

I agree with what SFaringer, Bruce and GHorn have outlined. I would examine the broken ends with a magnifying glass. Look for as George recommend to determine if it looks cut or fractured. I have doubts that both legs of a properly installed cotter key would fail on both legs. The short leg has the appearance of a slight downturn at the fracture. The rounded looped end has the appearance of having been slightly flattened by the installer. Having the downturn at the short leg where the broken off missing piece is makes me believe this likely occurred during removal from wherever it came from. If looking at the long leg where the break is, if the two pieces are put in alignment with one another I believe the cotter pin is to flattened to have come from a rod bolt. My thinking is that it was lost into the engine during a removal. Where it came from is anybody's guess. Without identifying that all possible cotter key's are in place would worry me to no end.

All that being said I have found items in the oil pick up screen twice . First was rod end bearing material from a Factory new Lycoming engine with 480 hours on it. Second a rivet from a Bendix old style riveted impulse coupling which had actually rattled and chaffed it's way trough the pick up screen passed up the suction path and jammed the oil pump gears shearing the drive coupling shaft and causing the the engine to stop running as the cam stopped rotating. I have to wonder how many hours that took? Fortunately it happened on short final on a twin engine Seneca. I use this last story to convince owners that removing the oil pick up screen is justified. It is interesting to me that owners often say that the pick up screen has never been removed for examination. The 480 hour Lycoming had never had is pick up screen removed for exam.
Regards,
Jim

Found half a cotter pin in the screen of an O-300 in my 58 172 many moons ago. A bit later needed to change a cylinder. When the mechanic pulled it off he said “Holy %*$#, I don’t believe it.” There was the other half of the cotter pin in the connecting rod bolt at the crank.

Rich