Low oil pressure in flight/stuck oil control ring

[SpamPilot]

Had to make a precautionary landing (at an airport) due to falling oil pressure.  Lost 3 quarts in one hour, oil pressure fell into the yellow but not below about 40 psi, 4 quarts left in the sump, still above Lycoming minimum of 2.  No external sign of oil loss, but plugs on #4 were fouled with oil.  Replaced the cylinder, found the old oil control ring was sticking in its groove, apparently due to varnish build-up.  Also noted scuffing around the piston crown, some old spots, some newer and still shiny.  Cerminil bore appears undamaged.

 

Do I trust the remaining cylinders, or do I do a preemptive top overhaul?  900 TSMOH

[ArtVandelay]

I would watch it like a hawk, oil analysis every change, pull plugs, scope it every 100 hrs, change the oil often, every 25 hrs or so, and then decide if based on what you see.

[Andy95W]

Like Ronald Reagan said: Trust but verify. You probably want to borescope the remaining cylinders within the next 10 hours.

I would trust the other cylinders but definitely have them on some sort of watch program of borescoping every 100 hours. Oil analysis probably couldn't hurt, either.

[jetdriven]

If you look carefully, it's not scuffing. It's torching. The gases blow around the piston, strip the boundary layer air off the piston and melt the edge of the crown. It looks like torch marks. Fwiw we just replaced the rings on a Piper Arrow (IO-360) for the same exact reason. Curiously, on #4 as well. No oil consumption changes, but 25 PSI on a compression check and plugs full of oil. The ring end gaps were all lined up as well.

Fly on. It's four one-cylinder engines all bolted to a common engine case. Monitor the others closely, and run at peak EGT at all power settings below 65%, including taxi and descent. It minimizes carbon in the rings which cause this.

[SpamPilot]

Actually, I did look closely.  In this case it is scuffing, not torching, but thanks for the suggestion.  The scuffing is at one specific point on the crown.  Vertical witness marks attest to the scraping action.  The normal carbon deposit ring around the bore at the top limit of piston travel has been scraped clean at this point.  The bore is CermiNil and appears undamaged.  Found a few slivers of aluminum in the oil control ring groove as well, though after removing them, the ring still sticks.

 

I would inline the pictures, but, infuriatingly, neither the "insert image" nor the "insert link" options work when posting a reply to this forum (dialog boxes don't respond to either "OK" or "Cancel").  Nor do the "Paste" or "Past as Text" links work either.  So I have attached pictures below.  You can also find them in the Gallery.

 

FYI, unless they have become stuck, rings normally rotate at about 1 Hz.  The gaps are always passing each other when the engine is running, so the fact that they are or are not aligned is immaterial.

[pinerunner]

If you look carefully, it's not scuffing. It's torching. The gases blow around the piston, strip the boundary layer air off the piston and melt the edge of the crown. It looks like torch marks. Fwiw we just replaced the rings on a Piper Arrow (IO-360) for the same exact reason. Curiously, on #4 as well. No oil consumption changes, but 25 PSI on a compression check and plugs full of oil. The ring end gaps were all lined up as well.

Fly on. It's four one-cylinder engines all bolted to a common engine case. Monitor the others closely, and run at peak EGT at all power settings below 65%, including taxi and descent. It minimizes carbon in the rings which cause this.

I'm glad to see someone else using peak EGT at low power settings, and giving a reason why.

[jetdriven]

Actually, I did look closely.  In this case it is scuffing, not torching, but thanks for the suggestion.  The scuffing is at one specific point on the crown.  Vertical witness marks attest to the scraping action.  The normal carbon deposit ring around the bore at the top limit of piston travel has been scraped clean at this point.  The bore is CermiNil and appears undamaged.  Found a few slivers of aluminum in the oil control ring groove as well, though after removing them, the ring still sticks.

 

I would inline the pictures, but, infuriatingly, neither the "insert image" nor the "insert link" options work when posting a reply to this forum (dialog boxes don't respond to either "OK" or "Cancel").  Nor do the "Paste" or "Past as Text" links work either.  So I have attached pictures below.  You can also find them in the Gallery.

 

FYI, unless they have become stuck, rings normally rotate at about 1 Hz.  The gaps are always passing each other when the engine is running, so the fact that they are or are not aligned is immaterial.

 

If you bead blast that black stuff off, like we did, it might be torching. FWIW all 4 of the pistons on the old engine we had on our J looked like this too.

 

The ring end gaps were all lined up and the cylinder blew 25/80.  In flight its irrelevant for power production, but it alerted us to pull the cylinder and the oil ring was frozen.  I might have flown it more and rechecked it, but thats me. the IA owned the airplane and said the cylinder is coming off.