Oil ring ?

Darren S · #1 10-08-2012, 09:10 AM

Can an oil ring be broken yet there be NO cylinder barrel scoring, NO high oil consumption and NO prematurely blackened oil ?

Compression is fine, oil analysis showed a spike in Iron and Chromium and most notably, the bottom plug keeps oil fouling. In flight parameters are fine, in flight mag check is fine.

Anyone ever seen this before?

Darren

aerhed · #2 10-08-2012, 10:22 AM

Yeah, an oily bore will show undeserved good compression. If your dead sure that plug is firing normally, then my guess would have to be the ring/bore interface. Maybe you've got 3 real good jugs masking a little oil consumption on the dud. It doesn't have to have a broken ring, and the problem very well may be part way down the bore. They don't have to be very far out of round to lose the seal. Sac Sky ranch has some pretty good pictures/discussion about cylinder bores. Picture's worth a thousand. You still haven't said what kind of cylinders you've got. Steel and nitrided will have chrome plated comp rings. Chrome & cermichrome, of course have chrome bores. I've seen rings go right through cermichrome into the steel with only a couple hundred hours. On 4 ringers you can have a scraper upside down.

rocketbob · #3 10-08-2012, 10:42 AM

The problem isn't in your rings, its your cylinder walls....specifically the roughness of the hone crosshatch.

walkman · #4 10-08-2012, 10:46 AM

So, is it possible to simply rehone the cylinders (assuming nitrided barrels) and reassemble with the same components? Is it worthwhile installing new rings at the same time?

aerhed · #5 10-08-2012, 11:10 AM

Yes, go with new rings. Examine the bore carefully after removal, before honing. Look for bore flaws, scores, skips, etc. If you see skips indicating the bore is not true, I wouldn't use a dingle ball hone. It'll just make it worse. Like Bob was saying, if its just the hone finish, that's easy. If your chrome spike in the oil is coming from this area with a steel jug, it may be the top ring(s) coming apart. If you re-ring and hone be sure to check end gap on the rings. Todays rings are so consistent they almost acts like a bore gage. Big gaps means you've got a big bore. On an 8.5:1 360 the top of the top ring stops at just about 6 3/8" from the bottom of the jug. If you use ECI rings the gap will be near minimum there with a normal choke.

Darren S · #6 10-08-2012, 04:04 PM

Thank you for the thoughts. It's a Superior engine put on the plane in 2006. Sorry I don't know what type of cylinders I have. I assume I have steel barrels, but I'm not sure. It's a garden variety IO-360 with 775 hours on it.

The consensus sounds like the jug needs to come off. I'm not one for pulling bolts unnecessarily but I'd rather take care of a problem sooner than later. I would put new rings and honing if that is indicated. Then I would have to break-in that jug again I figure.

Now for possible causes. The engine runs regularly, CHT's never over 400 degrees, I add Camguard to the oil to help decrease wear. The dang engine is babied and yet this still occured!! Is this just how these Lycoming are? No matter what you do a jug is going to have to come off before 1000 hours?

I usually keep the oil at 5 quarts and lean out any chance I get. This wouldn't have anything to do with a cause?

Thanks again for the clarification

Darren

rocketbob · #7 10-08-2012, 04:10 PM

Oil fouling of bottom plugs is not unusual, in fact its normal.

Russ McCutcheon · #8 10-08-2012, 06:46 PM

Quote:

Originally Posted by rocketbob

Oil fouling of bottom plugs is not unusual, in fact its normal.

Do you mean to say its normal or it’s common?

There is a difference. I would not say it’s normal, or maybe I got a good one, 1100 hours or so on the first run of a set of Lycoming angle valve cylinders, no oil ever shows on any of the plugs. AutoLite fine wire plugs.

If the plug is good I would fix the cylinder.

gasman · #9 10-08-2012, 06:57 PM

In 33 years of owning airplanes, I have never had an oil fouled plug. But then, I have never had a broken ring or a guide that was that bad.

I have also never had a plug foul from lead deposits...... maybe it's just luck.

rocketbob · #10 10-08-2012, 11:00 PM

You guys must not have ever owned an engine with chrome cylinders. Seeing oil pooled at the bottom of the cylinder after sitting a couple of days is the norm. Even if the cylinder is not chrome oil will accumulate at the bottom and will work its way to the lowest point which is the spark plug hole, especially if the crosshatch doesn't provide for enough surface tension to retain oil on the cylinder walls.

Darren you really don't have a problem that warrants disassembly.

The oil ring is there to effectively meter the amount of oil on the cylinder wall. Oil rings don't typically break because they're never exposed to the same amount of heat and pressure that the top two rings are.