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How can one detect a broken piston ring (high oil consumption)?

I went to the plane today to put some stuff in, and just checked the dipstick. On a quick look I burnt about 2qts on the last flight which was 6:10 airborne time. If true, that is a doubling from recent history… The oil has about 30-35hrs on it, so not too old.

We will service it tomorrow so will check for any oil leaks. Even a tiny leak will use up a lot of oil…

But how can one check the rings? The plugs don’t normally tell much of a story – their condition seems to depend on how the engine was leaned during the minute or two before the last shutdown.

Shoreham EGKA, United Kingdom

Compression tester is the easiest to try

Darley Moor, Gamston (UK)

Ideally use a leakdown (not compression) tester. Pretty low cost and easy to use. But beware of the cheapest ones that have very poor accuracy gauges and poor regulators. The Sealey one for example is junk. Hopefully you have previous leakdown results to compare against.

I can’t imagine this would indicate a problem with an oil control ring though.

And maybe a borescope down the spark plug hole to check for marks on the cylinder. Again, some fairly low cost options for these nowadays.

Any advice on this subject is like stepping into a minefield.

The advice about compression tests and oil consumption clues is good but at the end of the day you will never no for sure until you pull the pot off.

Yes, but which one?

Compression tests have always been really good on this aircraft – 78/80 or so. But these measure the upper ring only, and the valves. The oil getting into the combustion chamber is controlled by the bottom ring, which doesn’t get tested on the compression test – AIUI.

Shoreham EGKA, United Kingdom

Peter wrote:

Yes, but which one?

The one with the oily lower spark plug?

This is the face of hell. It happened to me in the middle of the Mediterranean and managed to land at Salerno with only a spoonful of black oil remaining.BTW compression Test was OK by the time .


What happened, @MedFlyer?

The one with the oily lower spark plug?

I always have some oily bottom plugs. IMHO, the spark plug condition depends mostly on how lean the engine was running just before the last shutdown.

I have just done the service with an A&P. Here are the 12 plugs.

Top 6 in order

Bottom 6 in order

All URHM38S Tempest iridium plugs.

The cylinder internals look OK. The criss-cross pattern is clear on all six.

IMHO the oil is just being burnt because it gets past the piston rings, for some reason. There is a significant deposit on the bottom of the lower cowling, which seems worse than in the past.

I have also had an interesting input from one experienced pilot/A&P which is that high altitude flight burns more oil, especially in an engine which has a fair bit of time on it, because the crankcase pressure has less external pressure to balance it, so more oil escapes via the breather. I have seen (with a very small sample size, but I will monitor it from now on) a correlation along these lines.

Shoreham EGKA, United Kingdom

The 2nd and last bottom plugs look very oily, unnormal in my experience.

Regarding the crankcase theory: I don’t believe it, not the amount of oil you lose. There would be a lot of oil everywhere on the belly and inside the elevator. Create some contraption at the crankcase breather that collects the oil and find out how much you lose that way.

@ Peter:
Well that was back in 2006.Bad combination of indifferent mechanic and hot summer as allways.I had allready reported high oil temperatures.Mechanic was insisting that a plugged oil cooler or a thermostat might be the problem since Compression Test was good. I started a long flight to Portugal.On the way,more and more oil was used per hour together with those high oil temps. Upon returning and west of Capri,suddenly horror time…..O oil pressure and Red hot oil Temp and those WW2 black smokes following my tail.I quickly decelerated to Vy to conserve the last margins of the Lyco and reached Salerno.The broken oil retaining ring is easily shown plus the black hot combustion vapors escaping up and down .

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